The Baltic Syneclise is one of the Paleozoic basins along the western margin of the East European Craton. Commercial amounts of hydrocarbons have been found onshore and offshore in the Middle Cambrian sandstones and Upper Ordovician limestone reefs. The Middle-Upper Cambrian and Tremadocian bituminous shales have been identified as good quality effective source rocks. The existence of good quality source rocks in the Ordovician and Silurian profiles provides an impetus for conventional and unconventional hydrocarbon explorations in this region. Geochemical analyses revealed that the rocks of the Middle–Late Ordovician and Early Silurian horizons exhibit overall good to very good source rock quality. Within the Ordovician strata, the Sasino and Prabuty formations exhibit the highest amounts of organic carbon with the median total organic carbon (TOC) values of 1.96 and 1.23 wt. %, respectively. The Pasłęk and Pelplin formations in the lowest parts of the Lower Silurian stand out clearly from other formations with the median TOC values of 0.91 and 1.15 wt. % and median total hydrocarbon content (S1+S2) of 2.46 and 1.54 mg HC/g rock, respectively. The analyzed successions are dominated by immature/early mature, algal (oil-prone) type II, and mixed II/I kerogen deposited in a marine paleoenvironment with anoxic conditions at the bottom and oxic conditions in the photic zone. Immature organic matter prevails in the eastern and central parts of the study area (Gołdap–Kętrzyn–Olsztyn area), and the western part (Darżlubie–Hel–Gdańsk area) has mature kerogen.

The Baltic Syneclise, in northeastern Poland, is located in the western part of the East European Craton (Figure 1(a)). It is one of the Neoproterozoic–Phanerozoic polygenetic sedimentary basins developed along the western slopes of the East European Craton and forms a large negative structure at the NW edge of the East European Platform (Figure 1(a)) [13]. The syneclise trends in the SW–NE direction and is 700 km (~1130 mi) long and 500 km (~800 mi) wide.

Hydrocarbons in the Baltic region have been investigated for over 200 years [4, 5]. The initial discovery of oil deposits in the Baltic Sea showed the presence of effective source rocks in the Baltic region. These source rocks include the Late Cambrian–Early Ordovician black shale called the Alum Shale [4, 612], which correlate with the Polish Piaśnica bituminous shales. The hydrocarbon accumulations in the Polish and Russian offshore and onshore parts of the Baltic Basin and southern Scandinavia, predominantly in Middle Cambrian strata, were sourced from bituminous shales such as the Piaśnica shales [6, 10, 1322].

Along with Alum Shale, good and excellent source rocks such as Piśnica, Sasino, Jantar, and Mingajny formations are present throughout the Early Paleozoic strata and are widespread in the Baltic Basin region [9, 10, 23, 24]. The increasing interest in the occurrence of unconventional hydrocarbon deposits has drawn attention to the remaining organic-rich shales, particularly in Late Ordovician and Early Silurian strata of the region [2529].

The aim of this study was to determine the organic matter quality of the Middle–Upper Ordovician and Lower Silurian horizons, their origin, and the vertical and spatial variability of thermal maturity of the organic matter. Rock–Eval pyrolysis, organic petrology, and biomarker analyses were applied to obtain information on organic matter richness, genetic type, and thermal maturity. The samples were also analyzed for paleoenvironmental conditions during organic matter deposition.

The Baltic Basin is situated along the western edge of the East European Craton and comprises several regional tectonostructural units (Figure 1(a)) [3032]. The Polish onshore part of this region is typically called the Peribaltic Syneclise, which practically refers only to the preserved Early Paleozoic deposits that presently form the giant depression. The thickness of the Early Paleozoic deposits to the east of the basin (Gołdap area) ranges from 500 to 1000 m (1640–3280 ft), increasing to 1500–2500 m (~4920–8200 ft) in the central part (Gdańsk and Bartoszyce areas) and to approximately 4000 m (~13,120 ft) in the west, close to the Teisseyre–Tornquist Zone (TTZ) [30, 33, 34]. The depositional profiles in the western part of the basin are widespread with considerably greater thickness than those in the eastern part. This is a consequence of higher subsidence rates, which consistently increased toward the TTZ [35]. The basin is bounded by the TTZ in the southwest, Baltic Shield in the north, and Mazurian–Belarus Anteclise along the southeastern margin (Figure 1(b)) [31]. Geologically, the Baltic Syneclise is a bridge between the East and West European platforms [5].

Numerous researchers, namely, Männil [36], Volkolakov [37], Witkowski [38], Vejbæk et al. [39], Šliaupa et al. [35, 40], Poprawa et al. [3, 41], Poprawa [42], ([43], and Karnkowski et al. [20], have comprehensively discussed the paleotectonic evolution of the Baltic Syneclise.

The initial stage of basin development was related to the break-up of the Precambrian Rodinia supercontinent during the Late Vendian–Early Cambrian periods [44, 45]. The present-day structural setting of the Baltic Basin resulted from the overlap of three principal tectonic stages: the Caledonian stage after the Silurian, Variscan stage after the Carboniferous, and Alpine stage after the Cretaceous [2, 34, 46]. Variscan deformation was of great significance because at the Carboniferous–Permian boundary, the present shape of the internal structure of the Baltic Basin was formed through the rejuvenation of Caledonian faults and accentuation of continuous and flexural deformations. The youngest Alpine deformations merely rejuvenated a part of the older dislocations. As per the tectonic expression, the Baltic Basin deposits can be divided into three structural complexes: Early Paleozoic, Late Paleozoic, and Permian–Mesozoic–Cenozoic.

The Early Paleozoic complexes in the study area comprise the lithostratigraphic section from the Ediacaran and Early Cambrian up to the Late Silurian and can be subdivided into four transgressive–regressive depositional sequences [47]. Sequence I includes the Late Ediacaran to Middle Cambrian deposits, except the uppermost part. This sequence represents continental deposits that grade upward into marine transgressive sandstones [30]. Sequence II comprises the upper Middle Cambrian to Early Tremadocian deposits. Sequence III is represented by the Arenig (Floian to lowermost Darriwilian) to Ashgill (Late Katian–Hirnantian) deposits, excluding the uppermost part. Lastly, sequence IV includes the uppermost Ashgill and Llandovery to Pridoli deposits.

The Cambrian rock series is known from several tens of boreholes in Poland, Russia (Kaliningrad region), and the Baltic countries (Lithuania, Latvia, and Estonia). The Cambrian consists of three distinct strata. The first is the Late Vendian–Early Cambrian continental sandstones and conglomerates [41] called the Żarnowiec Formation (Fm.) in the Polish part of the basin [4850], with thickness ranging from 10 to 200 m (~33–650 ft). The Żarnowiec Fm. traverses the Early Cambrian sandstones, siltstones, and sandstone–mudstone heteroliths of the Kluki Fm. This formation is strongly reduced or absent in the east, with maximum thickness reaching approximately 200 m (~650 ft) [30].

The Żarnowiec and Kluki formations are overlain by marine shelf sandstones of the Middle Cambrian Łeba Fm. [51], having a maximum thickness of 180 m (~590 ft). Black claystones, mudstones, and sandstones of the Sarbsko Fm. form the lower part of the Middle Cambrian section, reaching 200 m (~650 ft) [30, 52]. The upper part of the section is occupied by 80 m (~260 ft) thick gray sandstones and dark mudstones of the Dębki Fm. [51]. The uppermost part of the Middle Cambrian lithostratigraphic section comprises of poorly sorted quartz sandstones with phosphorite clasts of the Białogóra Fm. This formation occurs locally with a thickness not exceeding a few meters [30]. An erosional unconformity separates the Białogóra Fm. from the Late Cambrian (Furongian) deposits.

Late Cambrian deposits are known from two areas: the western part of the Baltic Syneclise [53] and the Łeba Elevation [48, 52, 54]. The Late Cambrian section is composed of black bituminous claystones with thin intercalations of two black limestone formations, Słowińsk and Piaśnica. The total thickness of these formations is of the order of 20 m (~65 ft).

In the eastern part of the Baltic Syneclise, the Middle and Late Cambrian–Early Tremadocian rocks have been significantly reduced or completely eroded [55, 56].

The Ordovician strata occur in stratigraphic and sedimentary continuity with the Late Cambrian strata; however, the present-day range of these strata is restricted by erosion, covering only a small fragment of the Baltic Syneclise [53, 57]. The topmost part of the black bituminous shales of the Piaśnica Fm. belongs to the Tremadocian (Figure 2(a)) [58]. This bituminous shale formation is covered by shales with glauconites of the Słuchowo Fm. (Figure 2(a)). This formation comprises dark gray and black shales with intercalations of gray-green shales with basal conglomerates. The Odargowo Limestone Member has been recognized within this formation from the Żarnowiec and Łeba areas and in the Baltic Sea (Figure 2(a)) [59]. The age of the formation was determined as Arenig (Floian–Early Darriwilian), with thickness varying from 3 to 20 m (~10–65 ft) [57]. The Słuchowo shale is overlain by marly limestones of the Kopalino Limestone Fm. (Figure 2(a)). The thickness of the formation, dated as Late Arenig–Early Llanvirn (Darriwilian), varies from <1 to approximately 30 m (3 to ~100 ft). This formation is further overlain by the Sasino Shale Fm. (Late Darriwilian–Early Katian) (Figure 2(a)), representing the deeper marine sediments, and comprises of black, dark gray, and green-gray shales, typically bituminous. The thickness of this formation reaches up to 35–70 m (~150–230 ft). The uppermost part of the Ordovician is occupied by the Prabuty Fm., dated at Ashgill (Late Katian–Hirnantian) (Figure 2(a)), which is composed of marls and shaly marls, gray mud-shales, and calcareous shales. In some sections, the uppermost part of the succession comprises quartz sandstone or sandy mud-shale beds [60]. The thickness of the formation varies from 3 to 20 m (~10–65 ft).

In the Baltic Syneclise, numerous boreholes have been drilled in the Silurian deposits. The presently applied Silurian lithostratigraphic scheme comprises seven formal units with five formations and two members [61] (Figure 2(b)). The Silurian deposits are represented by clay-shales and mud-shales and clay- and mud-shales with thin interbeds of fine-grained quartz sandstones [62]. Intraformational sedimentary gaps are absent in these sequences.

The uppermost Ordovician Prabuty Fm., partly eroded at the top, is unconformably covered by the Pasłęk Shale Fm. (Figure 2(a)) [61]. In the lowest part, this formation is represented by black bituminous transgressive shales of the Jantar Member (Figure 2(b)) belonging to the Llandovery and in some sections to the uppermost Ashgill Normalograptus persculptus zone [60, 63]. At the top of the section, the black shales grade into gray, green, gray-green, and black shales. In some boreholes, interbeds of brown-red and brown calcareous shales, marl and marly limestone beds, and bentonite laminae have also been observed.

The lower part of the Pelplin Shale Fm. (Figure 2(b)) [61] is dominated by dark gray, gray, and black shales; in some cases, calcareous interbeds and lenses of marly limestones can also be observed. The upper part is composed of gray and gray-green laminated shales, which are typically calcareous. Subordinate bentonite laminae occur within the shales. The lower part of the formation corresponds to the Early Wenlock, whereas the upper part is diachronic within various zones of Wenlock and Ludlow. The formation extends over the entire Polish part of the Baltic Syneclise; however, in the western part, near the Słupsk area, it thins away completely. As evident from the Żarnowiec IG-1 and Prabuty-IG 1 boreholes, it attains a maximum thickness of 400 m (~1315 ft).

The Pelplin Fm. gradually grades into the Kociewie Fm. (Figure 2(b)) [30], comprising gray and dark gray shales and calcareous shales, typically intercalated with gray siltstones, calcareous siltstones, and bentonite laminae [62, 64]. Shales and siltstones were formed within a vast deep-marine plain of the distal and medial parts of the shelf basin [64]. The thickness of this formation varies from 300 to 400 m (~980–1315 ft) near the Pasłęk, Gdańsk, and Żarnowiec areas to more than 3000 m (~9840 ft) in the Słupsk IG-1 borehole. In the upper part of the Pelplin Fm., the isochronous horizon of late Ludfordian Reda Member can be distinguished (Figure 2(b)) [65, 66]. The Reda Member comprises 10 to 30 m (~32–98 ft) thick calcareous siltstones and shale.

The Silurian succession ends with the Puck Fm. (Figure 2(b)) [61]. The lower part consists of shales and calcareous laminated gray to green shales. Bentonite laminae are sporadically observed grading upwards into laminated, gray-green calcareous shales, with marly and organodetritic light gray limestones and marls. The formation encompasses the uppermost Ludlow Hemsiella helmsiensis zone and the Pridoli. Entire successions of the formation are preserved in two areas: in the offshore borehole, where the thickness exceeds 900 m (~2950 ft), and in the Władysławowo and the Hel Peninsula areas, where it reaches approximately 1300 m (~4265 ft) [67]. Towards the south of this area, the thickness decreases to 300 m (~980 ft) in the Słupsk IG-1 borehole.

The total thickness of the Ordovician and Silurian strata in the Baltic Syneclise varies from <200 m (~655 ft) in the east to >3500 m (~11,500 ft) in the northwest of the syneclise.

3.1. Sampling

Samples from 19 boreholes located in the onshore eastern Polish part of the Baltic Syneclise were collected for organic geochemical study (Figure 1(b)). A total of 687 rock samples, representing the Middle–Late Ordovician and the Early Silurian strata, were sampled. The Ordovician strata are represented by 18 samples from the Kielno Formation. (Llanvirn–Caradoc age), 55 from the Sasino Formation (Llanvirn–Ashgill age), 14 from the Prabuty Formation (Aschgill age), and 11 from the Orneta Formation (Aschgill age) (Table 1); additionally, one sample each from the Pieszkowo and Kopalino formations was sampled.

The Early Silurian strata are represented by eight samples from the Barciany Fm. (Llandovery age), 183 from the Pasłęk Fm. (Llandovery age), and 398 from the Pelplin and Kociewie formations (Wenlock–Ludlow) (Table 2). The samples, typically represented by siltstones and claystone complexes and rarely by marls or silty limestones, were cleaned for mud contamination and crushed to the 0.5–2 cm (~0.2–0.8 in) fraction. For Rock–Eval pyrolysis and biomarker analysis, 200 g of each sample was milled to <0.2 mm (~0.008 in).

3.2. Rock–Eval Pyrolysis

Rock–Eval analyses were performed on all 687 core samples to obtain the basic geochemical parameters such as total organic carbon (TOC), hydrocarbon contents (S1 and S2), Tmax. These analyses were performed using a Rock–Eval II instrument equipped with a TOC module, based on the procedure described by Espitalié et al. [68]. For the best data analysis and interpretation, the reproducibility of the results of all samples was verified by replicate analysis.

The pyrolysis of organic matter was performed over a temperature range of 300–600°C with a temperature increase of 25°C/min. The parameters measured were free hydrocarbon content (S1), residual hydrocarbon content (S2), carbon dioxide produced during pyrolysis (S3), residual organic carbon content (S4), and Tmax temperature. These parameters were used to determine TOC, S2/S3 and S1/TOC ratios, production index (PI), hydrogen index (HI), and oxygen index (OI). Both the measured and calculated values provide a basis for the characterization of organic matter, including its quantity, genetic type, and degree of transformation [6971].

3.3. Extraction and Bitumen Separation

The 45 samples with the highest TOC and hydrocarbon contents, representing analyzed horizons and each borehole, were selected for further detailed geochemical studies. The selected samples were milled to a fraction size of <0.2 mm (~0.008 in) and extracted with dichloromethane/methanol (93 : 7, v/v) in a Soxtec and Soxhlet apparatus. The asphaltene fraction was then precipitated using n-hexane. Subsequently, the remaining maltenes were separated into compositional fractions of aliphatic hydrocarbons, aromatic hydrocarbons, and resins by column chromatography, using an alumina/silica gel (2 : 1, v/v) column (0.8×25cm) (~0.3–9.8 in). The fractions were eluted with n-hexane, toluene, and toluene/methanol (1 : 1, v/v).

3.4. Biomarker Analyses (GC–MS)

The isolated saturated hydrocarbon fractions were diluted in isooctane, spiked with 5β-cholane, and analyzed via coupled gas chromatography-mass spectrometer (GC–MS) for biomarker composition. The analysis was carried out with an Agilent 7890A gas chromatograph (GC) equipped with an Agilent 7683B automatic sampler, an on-column injection chamber, and a fused silica capillary column (I.D. 60m×0.25mm) (~197ft×0.01in), coated with 95% methyl/5% phenylsilicone phase (DB-5 ms, film thickness 0.25 μm). Helium was used as a carrier gas. The GC oven was programmed as follows: a temperature of 80°C was maintained for 5 min, increased to 120°C (248°F) at a rate of 20°C/min (68°F/min), increased further to 180°C (356°F) at a rate of 2°C/min (~36°F/min), and finally increased to 300°C (572°F) at 3°C/min (~37°F/min) and maintained for 35 min. The GC was coupled with an Agilent 5975C mass selective detector (MSD), which operated at an ion source temperature of 230°C (446°F), ionization energy of 70 eV, and cycle time of 1 s in a mass range from 45 to 550 daltons. The aromatic hydrocarbon fractions were diluted in toluene and analyzed by GC–MS using the same equipment as for the saturated hydrocarbon fraction. Orthoterphenyl was used as an internal standard. The GC oven was programmed as follows: a temperature of 80°C (176°F) was maintained for 1 min, increased to 120°C (248°F) at a rate of 20°C/min (68°F/min), increased further to 180°C (356°F) at 2°C/min (~36°F/min), and finally increased to 300°C (582°F) at 3°C/min (~37°F/min) and maintained for 35 min. The MSD was operated with a cycle time of 1 s in a mass range of 45 to 550 daltons.

The identification of individual compounds was made possible by identifying peaks on the chromatogram containing the desired fragmentation ions and characterized by the expected retention time.

3.5. Petrological Analysis

Petrological investigations were performed on a polished surface of the core sample using a Carl Zeiss Axio Imager A1m microscope equipped with a photometer PMT, computer with PMT III software, and HBO lamp. Rock materials were prepared according to the procedure recommended by Taylor et al. [72]. The reflectance measurements were carried out in nonpolarized light in oil immersion (refractive index n=1.518) and a 546 nm peak transmittance filter at a temperature of approximately 23°C (73°F). Prior to the measurements, the microscope was calibrated with the following standards: spinel (0.429%), yttrium–aluminum–garnet (0.905%), and gadolinium–gallium–garnet (1.728%). The microscopic investigations closely followed the guidelines published by ASTM [73]. Mean random reflectance readings were obtained on the population of graptolites, chitinozoans, vitrinite-like particles (VRg), and solid bitumen. The random reflectance readings for solid bitumen were calculated separately for the indigenous solid bitumen (VRb) and migrated solid bitumen (VRb2). The equivalent vitrinite reflectance values (EqVRo) calculated from the VRb and VRg measurements were based on the conversion formulas proposed by Jacob [74], Landis and Castaño [75], Petersen et al. [76], Waliczek et al. [77], and Luo et al. [78].

Specific components of organic matter were categorized according to the TSOP/ICCP (The Society for Organic Petrology/International Committee for Coal and Organic Petrology; [79]). The organic and mineral element compositions were determined by quantitative analysis of the organic component with a point-counting of 500 grains per polished section, as recommended by Taylor et al. [72]. Microphotographs of organic matter particles were obtained with a digital AxioCamMRc 5 camera.

4.1. Rock–Eval and Bitumen Extraction Results

Rock–Eval pyrolysis results indicate a large variability in the geochemical parameters and indices, which is observed within individual lithostratigraphic horizons and individual boreholes. The Middle and Late Ordovician strata are represented by samples from six formations: Pieszkowo, Kopalino, Kielno, Sasino, Prabuty, and Orneta. These formations have been dated at Llanvirn–Ashgill (Darriwilian–Rhuddanian) (Figure 2(a)). TOC values for the analyzed samples varied widely from traces to approximately 12.9 wt. % (Table 1). The highest organic carbon content was measured in the Prabuty Fm. (Early Ashgill); this stratum also exhibited the highest median value of 1.2 wt. %. Equally high contents of organic carbon and median values were observed for the black claystones of the Sasino Fm. The median of all the TOC in the analyzed Ordovician lithostratigraphic horizons was 0.8 wt. %. In contrast, the TOC values in the Pieszkowo, Kopalino, and Orneta formations did not exceed 0.1 wt. % (Table 1).

The hydrocarbon (HC) contents showed similar distribution and variability patterns. The S1 and S2 contents vary from traces to 6.66 mg HC/g rock and 25.28 mg HC/g rock, respectively. Some of the samples had limited hydrocarbon content. In over 30% of the analyzed population, the hydrocarbon contents were only in traces, <0.1 mg HC/g rock. The sum of S1 and S2 has been listed in Table 1.

The lowest TOC, S1, and S2 values were observed in the least frequently sampled Early Silurian Barciany Fm. Minor amounts of organic carbon and traces of hydrocarbons were observed within this formation (Table 2). The most frequently represented formations, Pasłęk (Llandovery) and Pelplin (/Kociewie) (Wenlock–Ludlow), are considerably rich in organic carbon and hydrocarbons. The TOC, S1, S2, and HC sum for the Pasłek Fm. varied from 0.1 to 14.61 wt. %, 0.04 to 5.33 mg HC/g rock, 0.13 to 73.72 mg HC/g rock, and 0.18 to 79.05 mg HC/g rock, respectively (Table 2). The Pelplin/Kociewie Fm. exhibits slightly less variability. Both the lithostratigraphic horizons present similar median TOC values, that of Pasłęk being 0.91 wt. %, whereas for Pelplin/Kociewie, it is 1.15 wt. % (Table 2); the median of S1 + S2 values for the two formations is 2.46 mg HC/g rock and 1.54 mg HC/g rock, respectively (Table 2).

HI, which describes the hydrocarbon potential [69, 70], is interpreted as an indicator of the ability of organic matter to generate hydrocarbons. The higher the HI value, the better is the quality of the source rock. For geochemical characterization, the HI index is also used to determine the type of kerogen. In the present study, the HI index showed variability similar to the TOC and HC contents. This is an evident consequence of the fact that this index is calculated as the quotient of the TOC and S2 values [69]. Samples with <0.2 wt. % TOC typically do not provide reliable HI values because of the low amounts of S2 hydrocarbons.

The observed HI variability in the Late Ordovician and Early Silurian rocks is as follows: 34–144 mg HC/g TOC in the Kielno Fm., 30–508 mg HC/g TOC in the Sasino Fm., 80–873 mg HC/g TOC in the Prabuty Fm., 8–901 mg HC/g TOC in the Pasłęk Fm., and 14–448 mg HC/g TOC in the Pelplin/Kociewie Fm. (Tables 1 and 2). Despite the significant variability, the median of HI in analyzed horizons is similar: 74 mg HC/g TOC in Kielno Fm., 109 mg HC/g TOC in Sasino Fm., 126 mg HC/g TOC in Prabuty Fm., 133 mg HC/g TOC in Pasłęk Fm., and 140 mg HC/g TOC in Pelplin/Kociewie Fm. (Tables 1 and 2).

The Rock–Eval Tmax measurements were used to assess the degree of maturity of the organic matter. Samples with <0.2 wt. % TOC do not provide reliable Tmax values. Therefore, the number of reliable Tmax measurements in relation to the total number of samples is lower, merely 557 measurements (Tables 1 and 2), which is 80% of the analyzed population of samples. Half of the Tmax values did not exceed 430°C, and only 5% exceeded 440°C. The medians of Tmax for individual formations are presented in Tables 1 and 2. The results clearly indicate that the organic matter in the Ordovician and Silurian rocks is immature or with low maturity. Single, elevated values of 458°C (Pasłęk Fm. in Hel IG-1 and Gdańsk IG-1 boreholes) and 468°C (Pasłęk Fm. in Hel IG-1 and Pelplin Fm. in Prabuty IG-1) were not confirmed by the EqVRo values. Another useful indicator for indirectly assessing the degree of maturity is the production index (PI=S1/S1+S2). PI values above 0.4 indicate the predominance of epigenetic hydrocarbons, which in turn indicate the initiation of the hydrocarbon generation processes. The median of PI values is generally below this value, but can reach 0.82 in certain lithostratigraphic horizons such as Sasino Fm., 0.62 in Prabuty Fm., and 0.75 in Pelplin/Kociewie Fm. (Tables 1 and 2).

The 45 samples with the highest of organic matter content (TOC values 3–6%), which represented the main lithostratigraphic horizons, were selected for bitumen extraction and compositional fractional analysis; the bitumen extract content varied over a wide range, from 170 to 22,533 ppm (Table 3). Both values were obtained for the samples from Pasłęk Fm. The detailed extraction results are listed in Table 3. The fractional composition of the samples was also highly variable; however, the saturated and aromatic hydrocarbons predominated (Table 3).

4.2. Biomarker Analyses

Biomarker analyses were performed on 25 samples from the Sasino (six samples), Prabuty (two samples), Pasłęk (eight samples), and Pelplin/Kociewie (nine samples) formations.

4.2.1. Aliphatic Fraction

All analyzed samples contained compounds, such as n-alkanes, acyclic isoprenoids, terpanes, and steranes, that were identified to define the paleoenvironmental conditions, origin, and thermal maturity of the organic matter.

The analysis of fragmentation ion m/z 71 revealed the presence of n-alkanes from C14 to C38 homologues. In all analyzed samples, n-alkanes had a monomodal distribution with the dominant role of short-chain compounds. C17 and C18, n-alkanes were abundant in all samples except for two cases where C19 and C20 dominated at m/z 71 fragmentogram: Darżlubie IG-1/2936.8 (Prabuty Fm.) and Hel IG-1/2951.8 (Pasłęk Fm.) samples (Table 4).

Two indicators were calculated based on the n-alkane distributions, carbon preference index (CPI), and terrigenous/aquatic ratio (TAR). CPI values vary from 0.89 to 1.08 (Table 5) and are dependent on thermal maturity and paleoenvironmental conditions. Samples from Sasino Fm. are characterized by CPI results varying between 0.94 and 1.07. Two samples representing the Prabuty Fm. have CPI values of 1.08 (Table 5). The CPI results from Pasłęk Fm. vary between 0.89 and 1.08, whereas those from Pelplin Fm. vary between 0.89 and 1.05.

TAR differentiates the origin of organic matter, with values below 1 indicating marine origin of the organic matter, whereas those above 1 suggesting terrigenous origin [80]. TAR values for almost all analyzed samples were below 0.3, which suggest that short-chain n-alkanes dominate above long-chain homologues (Table 4). Only one sample from the Pasłęk Fm. (Hel IG-1/2951.8 sample) has a value of 1.53.

Two acyclic isoprenoids, pristane (Pr) and phytane (Ph), were also marked on m/z 71 chromatograms. The Pr/Ph ratio describes paleoenvironmental redox conditions. In addition, the juxtaposition of the Pr/n-C17 and Ph/n-C18 ratio helps in determining the maturity and origin of the organic matter. The Pr/Ph, Pr/n-C17, and Ph/n-C18 ratios vary within the Sasino Fm. from 0.39 to 1.20, 0.10 to 0.36, and 0.27 to 0.42, respectively (Table 4). The results of the three ratios for the Prabuty Fm. are between 0.86 and 1.20, 0.23 and 4.51, 0.39 and 2.60, respectively. Pr/Ph, Pr/n-C17, and Ph/n-C18 values within the analyzed Pasłęk Fm. samples varied from 0.15 to 1.10, 0.18 to 0.67, and 0.22 to 0.72, respectively, whereas those for the Pelplin Fm. varied from 0.44 to 1.61, 0.13 to 1.33, and 0.26 to 0.88, respectively (Table 4).

Sterane distributions were identified on the m/z 217 ion chromatograms, and αββ steranes were marked on m/z 218 ion chromatograms. The combined results of these two mass ion chromatograms allowed us to calculate the following indicators: regular ααα sterane distribution, C27 diasteranes/(diasteranes+regular steranes) (C27 dia/(dia+reg)), C29 S/(S+R), and C29ββ/(αα+ββ).

The distribution of regular ααα steranes revealed that C27 regular steranes have a dominant role in almost all cases except the two samples from Kętrzyn IG-1 borehole (Pasłęk and Pelplin formations), where regular ααα C29 steranes have maximum abundance. The results of C27 dia/(dia+reg) ratios, linked with paleoenvironment conditions, vary from 0.37 to 0.46 in Sasino Fm., 0.41 to 0.47 in Prabuty Fm., 0.17 to 0.49 in Pasłęk Fm., and 0.21 to 0.53 in Pelplin Fm. (Table 4). C29 S/(S+R) and C29ββ/(αα+ββ) indicators are linked with thermal maturity of organic matter, and in all samples, except those from the Kętrzyn IG-1 borehole, the results for these indicators are high. The C29 S/(S+R) ratio displays values ranging from 0.38 to 0.52 for the Sasino Fm., 0.40 to 0.53 for the Prabuty Fm., 0.17 to 0.50 for the Pasłęk Fm., and 0.19 to 0.62 for the Pelplin Fm. The C29ββ/(αα+ββ) values vary from 0.52 to 0.60 for samples within Sasino Fm., 0.56 to 0.58 within the Prabuty Fm., 0.39 to 0.62 within the Pasłęk Fm., and 0.38 to 0.65 within the Pelplin Fm. (Table 5).

Furthermore, terpanes have also been marked on m/z 191 ion chromatograms. The analysis of these compounds allows the calculation of several maturity indicators: moretane/hopane (Mor/Hop), C31 S(S+R), and Ts/(Ts+Tm) (Table 5). Mor/Hop values for the Sasino Fm. varied from 0.06 to 0.19. The results of this ratio for the other formations range from 0.13 to 0.19 for the Prabuty Fm., 0.13 to 0.47 for the Pasłęk Fm., and 0.13 to 0.41 for the Pelplin Fm. C31 S(S+R) results were between 0.56 and 0.65 for the Sasino Fm., 0.60 and 0.61 for the Prabuty Fm., 0.27 and 0.63 for the Pasłęk Fm., and 0.26 to 0.63 for the Pelplin Fm. Ts/(Ts+Tm) values varied from 0 to 0.66 for the Sasino Fm., 0.23 to 0.49 for the Pasłęk Fm., and 0.27 to 0.59 for the Pelplin Fm.; for the Prabuty Fm. samples, the values were equal to 0.47 (Table 5).

C29H/Hop, which is a paleoenvironmental indicator, was calculated based on the terpane distribution. The calculation of triterpane indicators in some samples was impossible as triterpanes were not detected. C29H/Hop results vary from 0.60 to 0.93 in the Sasino Fm., 0.80 to 0.92 in the Prabuty Fm., 0.67 to 0.89 in the Pasłęk Fm., and 0.52 to 1.02 in the Pelplin Fm. (Table 4). Homohophane distributions were analyzed to evaluate the redox conditions. Tetra- and pentahomohopanes were present in most samples except Kętrzyn IG-1/1516.6, Kętrzyn IG-1/1493.6, Darżlubie IG-1/2956.9, and Hel IG-1/2990.1.

The regular sterane/17α hopane ratio was calculated to comprehensively define the origin of organic matter. Values below 1 indicate bacterial input, whereas those above 1 suggest that algal input exceeded bacterial input. The values of regular sterane/17α hopane ratio vary from 0.6 to 3.2 in the Sasino Fm., 0.82 to 1.18 in the Prabuty Fm., 0.55 to 2.65 in the Pasłęk Fm., and 0.5 to 2.25 in the Pelplin Fm. (Table 4).

Gammacerane was absent or noted in very low abundances. β-Carotane was not found in any sample.

4.2.2. Aromatic Fraction

Aromatic hydrocarbons were present in all analyzed samples. Identified compounds were collected in Table 6. The quantity of compounds listed in Table 6 was used to calculate a few indicators.

The dimethylnapthalene ratio (DNR) based on dimethylnaphthalenes (DMN) exhibited variability from 1.13 to 3.02 in the Sasino Fm., 1.13 to 2.47 in the Prabuty Fm., 1.43 to 4.09 in the Pasłęk Fm., and 1.66 to 3.66 in the Pelplin Fm. (Table 5). The trimethylnapthalene ratio (TNR1) based on trimethylnaphthalenes (TMN) varied from 1.12 to 1.46 in the Sasino Fm., 1.16 to 1.47 in the Prabuty Fm., 1.03 to 1.24 in the Pasłęk Fm., and 0.86 to 1.22 in the Pelplin Fm. (Table 5).

Methylphenanthrene index-1 (MPI-1), based on methylphenanthrene distributions, was recalculated to RcalMPI1 in the Ro vitrinite reflectance equivalent scale. Calculated values of RcalMPI1 exhibited variations from 0.96 to 1.40 in the Sasino Fm., 1.10 to 1.16 in the Prabuty Fm., 0.78 to 1.35 in the Pasłęk Fm., and 0.92 to 1.12 in the Pelplin Fm. (Table 5).

Dibenzotiophene/phenantrene ratio (DBT/Phen) in all samples was below 1 and reached values from 0.13 to 0.67 in the Sasino Fm, from 0.16 to 0.62 in the Prabuty Fm, from 0.03 to 0.64 in the Pasłęk Fm., and from 0.09 to 0.23 in the Pelplin Fm. (Table 5).

Okenane, isorenieratane, β-carotane diaryl isoprenoids, and aryl isoprenoids were absent in most of the analyzed samples. Diaryl isoprenoids were observed only within the samples from the Kętrzyn IG-1 borehole. The presence or absence of other analyzed aromatic compounds was used to determine the origin and maturity of the organic matter. These compounds prove useful in describing the paleoenvironmental conditions.

4.3. Petrological Analyses

Petrological investigation was conducted on 25 samples representing the Sasino (six samples), Prabuty (two samples), Pasłęk (seven samples), and Pelplin (nine samples) Formations. The microscopic analysis included reflectance measurements and the determination of organic matter components.

4.3.1. Reflectance Measurements

The mean random reflectance measurements are presented in Table 7, and selected histograms of the reflectance values are shown in Figure 3.

The observed mean solid bitumen reflectance values for the Pelplin, Pasłęk, Prabuty, and Sasino Formations were 0.80–1.04%, 0.71–1.02%, 0.88–0.92%, and 0.84–0.89%, respectively. For a sample from the Pasłęk Fm. (Hel IG-1/2951), the solid bitumen reflectance may be unreliable owing to the low number of single reflectance measurements. The highest VRb values, at approximately 1.02% Rb, were observed in four samples, two from the Pelplin Fm. (Prabuty IG-1 borehole) at a depth of approximately 3300 m (~10.830 ft) and two from the Pasłęk Fm. (Gdańsk IG-1 borehole) at a depth of approximately 3080 m (~10.100 ft). The lowest value of VRb (mean 0.73%) was observed in the two samples from the Pasłęk Fm. (Hel IG-1 borehole) at a depth of 2951 m (~9680 ft). In the Prabuty IG-1/3328 sample, the mean reflectance of pyrobitumen was measured at 1.46% Rb2.

The mean reflectance values of graptolite, chitinozoa, and vitrinite-like particles (VRg) vary as 0.54–1.16%, 0.51–1.23%, 1%, and 0.59–1.06% in samples from the Pelplin, Pasłęk, Prabuty, and Sasino Formations, respectively. The highest VRg values were consistent with the highest VRb measurements.

The vitrinite reflectance equivalent (EqVRo) for the analyzed samples was calculated using five different empirical formulas (Table 8).

The equivalent vitrinite reflectance calculated from VRb using the procedure stated by Jacob [74] and Waliczek et al. [77] provided similar results as recalculated VRg values using the Petersen et al. [76] formula and varied between 0.53 and 1.06% EqVRo. Higher recalculated reflectance values, ranging from 0.58 to 1.33% EqVRo, were obtained using Landis and Castaño [75] and Luo et al. [78] formulas.

4.3.2. Organic Matter Composition

The organic matter composition has been summarized in Table 9, and the selected organic components are shown in Figure 4.

The main constituent in the studied samples was solid bitumen (Figures 4(a) and 4(h)), accounting for 1.0–6.2% (mean 2.8%) of the contents. Solid bitumen was not recognized in the two samples from the Kętrzyn IG-1 borehole and the one from the Olsztyn IG-2 borehole. The second dominant type of organic matter is graptolite (average 1.7% vol., ranging from 0.2 to 7.6% vol.) (Figures 4(g) and 4(i)), unidentified only in five samples. The content of the liptinite group, excluding samples from the Pelplin and Pasłęk Formations (Kętrzyn IG-1 borehole) and one sample from the Sasino Fm. (Olsztyn-IG-2 borehole), varies from 0 to 3.4% for alginite and from 0.5 to 4.8% for liptodetrinite. Macerals from the liptinite group are dominated by major bituminite in samples from Kętrzyn IG-1 (18.3%) (Figure 4(b)) and with Tasmanites algae in Olsztyn IG-2 (10.6%) (Figures 4(d) and 4(e)) boreholes. In some samples, organic matter is present in the form of groundmass with mineral matrix classification as well as bituminite association. Chitinozoa particles were observed in the Darżlubie IG-1/2956.9 sample.

Other organic matter components similar to vitrinite (possible phytoclast) or zooclasts, whose classification is inconclusive, have been defined as vitrinite-like particles. The content of such organic matter within the analyzed samples varied from 0 to 6.8%, with a mean value of 1.8% vol.

5.1. Quality of Organic Matter from Rock–Eval and Extraction Data

The organic matter quality analysis of Middle–Late Ordovician and Early Silurian rocks was conducted on the clayey horizons. Carbonate horizons, such as Pieszkowo, Kopalino, Barciany, and Orneta formations, in which the preliminary Rock–Eval studies did not show organic carbon content or were at the level of hundredths of a percent are not considered in this study. The exceptions are limestones from the Kielno Fm., where the levels with relatively high TOC contents were measured.

The studies showed that Ordovician rocks have a source quality similar to that of the Late Cambrian–Early Ordovician. Although the analyzed clay-mud formations are not as homogeneous and high in organic carbon and hydrocarbon content as the Piaśnica bituminous shale, very high to even excellent source qualities have been reported within them (Figure 5). These elevated levels represent the shales of the Prabuty and Sasino formations, which are reasonably organic-rich (up to 12.9 and 10.6 wt. %, median 1.2 and 1.9 wt. %, respectively) and hydrocarbon-rich (up to 18.9 and 28.4 mg HC/g rock, with median of 8.1 and 6.5 mg HC/g rock, respectively) (Table 1). These samples are considered examples of good to very good source rocks (Figure 5); the Caradoc black shale is also known from adjacent areas. Excellent source rocks have been reported from Lithuania and onshore Kaliningrad, where TOC contents may exceed 6 wt. % [10, 12, 25, 28, 81, 82]. The TOC values in this range are considered as “good to very good” [83]; it is possible that the previous research overlooked the significance of this formation due to the existence of the prolific bituminous shales of the Piaśnica Fm. This reduced interest may also have resulted from the heterogeneity of these formations. The rich horizons coexist with poor and fair intercalations, where organic carbon content is below 1 wt. % (Figures 5 and 6).

The samples exhibit relatively high bitumen concentrations, up to 11,180 ppm (Table 3), with the exception of one sample from the Sasino Fm. of the Lidzbark Warmiński 1 borehole, which yielded 277 ppm of bitumen (Table 3). The correlation between TOC and bitumen concentrations, in Hunt’s [84] and Leenheer’s [85] correlation, shows that the analyzed samples have at least good oil-prone potential (Figure 7). Largely, the bulk composition of these bitumen extracts has yielded high saturate and aromatic hydrocarbon contents, >50%. However, the same samples show high polar and asphaltene fractional contents exceeding 50%; for example, Lidzbark Warmiński 1, Dębowiec Warmiński 1, and Lesieniec 1 boreholes have polar and asphaltene fractional values of 67%, 54%, and 67%, respectively (Table 3). The saturate-to-aromatic ratios are variable, but typically above 1; only two cases exhibit ratios below 1.

Despite being typically good geochemical indications of source rock, the shales of the Prabuty and Sasino Formations are not a confirmed source of conventional hydrocarbon accumulations, but can be rather a source of unconventional accumulations in zones of higher thermal maturity, e.g., west of the Gdańsk–Malbork zone.

Among the analyzed Middle and Late Ordovician formations, limestones from the Kielno Fm. alone showed small amounts of organic matter. The measured organic carbon content was small and exceeded 0.5 wt. % only for three cases, with the S1 and S2 hydrocarbon content being of the order of 0.3–0.6 mg HC/g rock (excluding one sample from Olsztyn IG-2 borehole with value 1.1 mg HC/g rock) (Table 1).

The Early Silurian mudstones and shales of the Pasłęk and Pelplin formations (Pelplin/Kociewie Fm.; the two formations are undivided in the Gdańsk IG-1 borehole) exhibit good geochemical properties. The most prolific bituminous shales of the Jantar Fm. are absent in the study area. Their stratigraphic equivalent, the limestones of the Barciany Fm., do not exhibit source rock character and have TOC values considerably <0.5% (Figure 5, Table 2).

The majority of the Pasłęk and Pelplin (/Kociewie) samples showed moderate organic carbon contents (0.5%<TOC<2%), except for a few samples with TOCvalues<0.5% and those with high TOC contents up to 14.6 wt. % (Figure 5, Table 2). The hydrocarbon (S1 and S2) contents are remarkably consistent with TOC and as high as those in the Ordovician Sasino and Prabuty formations. The hydrocarbon contents in the Pasłęk and Pelplin formations reach up to 79 mg HC/g rock and 9.2 mg HC/g rock, respectively (Figure 5, Table 2).

The bitumen concentrations in the Silurian formations are relatively high, up to 22,533 ppm (Figure 7, Table 3), with the exception of a few samples from the Pasłęk and Pelplin formations (Prabuty IG-1 borehole) that yielded lesser amounts of bitumen (170–680 ppm). Largely, the bulk composition of these bitumen extracts have high polar and asphaltene contents, >50%. However, the saturate fractions range from a few percent to 68%, and the aromatic fractions range from 0 to 52% (Table 3). Saturate-to-aromatic ratios above 1 were observed for most samples. Higher values of this ratio were observed in samples from the western part of the study area.

The HI values within both the Ordovician and Silurian formations show considerable variation, but with rather low average values, ranging 100–140 mg for the Sasino, Prabuty, Pasłęk, and Pelplin formations (Figure 8, Tables 2 and 3). This indicates a relatively poor hydrocarbon potential.

5.2. Origin of the Organic Matter

The analyses of the Rock–Eval pyrolysis results, biomarker distributions of aliphatic and aromatic fractions, and microscopic investigations revealed the origin of organic matter.

The Ordovician and Silurian source rocks contain relatively homogeneous type II and/or I kerogens. The presence of type III kerogen, suggested by the HI/Tmax correlation (Figure 8), is evidently impossible. The position of these samples in the type III kerogen field is due to poor preservation of organic matter or secondary processes. Such indications may be the result of the presence of hydrogen-depleted kerogen, which is indicated by relatively low HI values (Tables 1 and 2) or the influence of secondary processes such as oxidation. The humic organic matter formed a major constituent of the source rocks after the evolution of higher land plants in Late Silurian, which should be considered when interpreting the Rock–Eval data.

According to the n-alkane distribution, organic matter has a marine origin (Figure 9). In analyzed samples, the most abundant n-alkanes were n-C17, n-C18, and n-C19 (Table 4). This type of n-alkane distribution is associated with the phytoplankton-origin organic matter from the open sea [86, 87]. The phytoplankton origin of the Ordovician–Silurian organic matter from the study area was proposed by Skręt and Fabiańska [88]. The TAR values below 0.3 substantiate the marine origin; however, the Hel IG-1/2951.8 sample from the Pasłęk Fm. is an exception, which had TAR values above 1. This result seems to be unreliable as the biodegradation processes might be responsible for the high TAR results [80]. The marine origin of organic matter is also supported by the sterane distribution (Figure 10).

According to the triangle diagram of regular steranes (Figure 10), all samples, except those from the Kętrzyn IG-1 borehole, have an algal-phytoplankton origin. C27 steranes, linked with red algae, play a dominant role in these samples [89]. The sterane distribution in the Kętrzyn IG-1 borehole (Pelplin and Pasłęk Formations) (Table 4) exhibited a higher content of C29 steranes, which can be linked to a higher content of green-algae and/or terrigenous organic matter admixture [9092]. A higher contribution of C29 steranes was also observed in some Silurian samples obtained from the Baltic Sea and Lithuania area [11]. Organic matter in the Pelplin and Pasłęk formations from the Kętrzyn IG-1 borehole have algal/plankton origin with potentially little terrigenous input. The presence of 1-methyl-7-isopropylphenanthrene (retene) in the Silurian deposits and its link with primitive bryophytes supports this hypothesis [93]. Moreover, samples from the Kętrzyn IG-1 borehole contain perylene. The presence of this compound indicates that these samples contained immature/early-mature organic matter. Perylene is typically linked with fungi, which can be the next group of organisms living in areas adjacent to the Baltic Syneclise.

To evaluate the content of bacterial and algal input in the analyzed samples, a regular sterane/17α hopane ratio was calculated. As steranes are linked with algal input, results above 1 indicate an advantage of algal over bacterial input [94]. Hopanes are linked to bacterial inputs. The ratio of steranes/17α hopanes increases with increasing thermal maturity [95]. Previous surveys revealed that lithology also has an influence on the ratio as the lower values were observed in carbonates from low paleolatitudes; marine siliciclastics from higher paleolatitudes were typically characterized by higher sterane/17α hopane values [96]. Other studies have linked lower values of this ratio with terrigenous input or with microbially reworked organic matter [94, 97, 98]. Redox conditions play a crucial role in preserving the original sterane/17α hopane ratios [99]. When deposition occurs under anaerobic conditions, the ratio should provide proper results. In cases where aerobic degradation was observed, the ratio might be underestimated, as during the destruction of steroids, the heterotrophic and methylotrophic bacteria produce hopanoids [99].

In the Sasino Fm., algae dominated over bacterial input in four of the six analyzed samples (Table 4). The bacterial input within this formation prevails only in the samples from the Darżlubie IG-1 and Gdańsk IG-1 boreholes. However, higher Pr/Ph values in samples from the Gdańsk IG-1 borehole might suggest that the sterane/17α hopane ratio result is slightly underestimated by steroid degradation under more aerobic conditions [99]. All Ordovician samples contain a wide distribution of cyclohexylalkanes (characteristic ion m/z 82), which might be interpreted as a Gloeocapsomorpha prisca algae fingerprint [100].

In the Prabuty Fm., algal input prevails above the bacterial input, which is visible in the sample from the Hel IG-1 borehole. The second sample from the Darżlubie IG-1/2936.8 exhibits a situation opposite to that of the Hel IG-1 borehole (Table 4); however, the higher Pr/Ph result in this sample suggests that the redox conditions might have caused the sterane/17α hopane ratio.

In the Silurian Pasłęk Fm., the bacterial input had a greater role than algae in organic matter origin. One exception for this formation is the sample from the Kętrzyn IG-1 borehole, where algal input prevails over bacterial.

In the Pelplin Fm., the bacterial input dominates in the Prabuty IG-1 and Gdańsk IG-1/3035.8 boreholes (Table 4). The predominance of algal input was observed in the Darżlubie IG-1, Gdańsk IG-1 (sample 2926.2 m [~9600 ft]), Hel IG-1, and Kętrzyn IG-1 boreholes. The maximum dominance of algal input was observed in the Kętrzyn IG-1 borehole (Table 4). Algal input played an integral role in the Ordovician Period. The Silurian samples exhibited predominantly bacterial input; however, those in the Kętrzyn area exhibited strong algal dominance.

The lack of okenane, isorenieratane, β-carotane diaryl isoprenoids, and aryl isoprenoids in all samples, except those from the Kętrzyn IG-1 borehole, indicate that the bacterial input is not linked with purple and/or green sulfur bacteria. Some diaryl isoprenoids were observed in the m/z 133 ion chromatograms (Figure 11(a)) of both samples from this borehole. This may be evidence of the presence of purple and/or green sulfur bacteria.

All samples contained dehydroabietic acid methyl ester (DAME), which is a compound normally linked to conifers [101]; its presence is unusual in the Ordovician and Silurian samples. DAME in Early Paleozoic samples may be a reflection of cyanobacterial activity [102].

Microscopic investigations confirmed the marine origin of the organic matter in the analyzed Silurian and Ordovician samples. Twenty samples exhibited the characteristics of marine sediments, such as graptolites (Figures 4(g) and 4(i)). This group of zooclasts is considered highly gas-prone, with substantial hydrocarbon potential, associated primarily with kerogen type II/III [78], with graptolites having little or no contribution in hydrocarbon generation [103]. However, the liptinite group in most of the analyzed samples was represented by alginite macerals (Figures 4(c) and 4(f)) related to kerogen type I/II [104]. For three samples (Kętrzyn IG-1 and Olsztyn IG-2 boreholes) where graptolites were not observed, the abundance of telalginite in the Tasmanites acts as (Figures 4(d) and 4(e)) an indicator of a marine paleoenvironment [105]. The significant algal input in the sample from Kętrzyn IG-1 was also confirmed by the high sterane/hopane ratio (Table 4). Less frequent alginite and liptodetrinite with the highest thermal maturity were observed in all samples (Prabuty IG-1, Pelplin Fm.; Gdańsk IG-1, Pasłęk Fm.; Darżlubie IG-1, Sasino Fm.). In each sample, excluding the Kętrzyn IG-1 and Olsztyn IG-2 boreholes, the thermally altered solid bitumen was present as a secondary product [106, 107] (Figures 4(a) and 4(h)). This component is a primary-oil solid bitumen (VRo~0.7–1.0%) or late-oil solid bitumen (VRo~1.0–1.4%) [108] and is related to oil or the beginning of the wet gas phase. Bituminite [109], a possible degradation product of algae, bacteria, and zooclast, appears in six samples. It occurs as an amorphous groundmass in mineral matter of the Pasłęk (Gdańsk IG-1/3087.5; Hel IG-1/2966.6), Prabuty (Hel IG-1/2982), and Sasino (Gdańsk IG-1/3096.2) formations and in samples from the Kętrzyn IG 1 borehole (Figure 4(b)), as dark, brown streaks related to oil-prone organic matter. The high bituminite and Tasmanite contents (Table 9) in samples from Kętrzyn IG-1, as well as high sterane/17α hopane ratios, higher C29 sterane group content (Table 4; Figure 10), and abundant bitumen extract (Table 3) in samples at depths of 1516.6 m (~4975 ft) indicate highly oil-prone, mostly green-algae origin organic matter, dispersed in this potentially very good source rock.

5.3. Paleoenvironmental Conditions

Biomarkers are useful in determining paleoenvironmental conditions. In samples with high bacterial input, the homohopane distribution is a good biomarker for paleoenvironmental conditions. Homohopane distributions with high percentages of C34 and C35 homologues (Table 4, Figure 11(b)) indicate that anoxic conditions are common in the analyzed samples. Thermal maturity influences the distribution of homohopanes [110]. Pr/Ph is the second indicator typically used to define redox conditions. However, this ratio is vulnerable to lithology, considering that its results in carbonates might be underestimated, and salinity levels as higher abundance of phytane might be linked with halophilic bacteria [111, 112]. Furthermore, pristane concentrations might be significantly higher in the case of algal tocopherol additional input [88]. As mentioned above, the Pr/Ph results were juxtaposed with homohopane distributions.

According to homohopane distribution, suboxic redox conditions were observed in Prabuty IG-1/3328.5 and in Hel IG-1/2966.6 samples from the Prabuty and Pasłęk formations, respectively. Homohopane distribution suggests oxic conditions only in the Darżlubie IG-1/2956.9 sample from the Sasino Fm. However, minor bacterial input in this sample, supported by the sterane/17α hopane ratio, might cause a lack of C32-C35 homohopanes [113]. Conclusions about redox conditions during deposition were confirmed by the Pr/Ph ratio, which in most samples did not exceed 1. In addition, high values of TOC support anaerobic conditions.

The juxtaposition of C27 sterane and diasterane ratio opposite to the Pr/Ph ratio indicates that the pH and Eh of the paleoenvironment were similar in all analyzed samples. According to this dependency, slightly more anoxic conditions were observed only in samples from the Kętrzyn IG-1 borehole.

The comparison between dibenzothiophene/phenanthrene (DBT/Phen) and Pr/Ph ratios (Table 4) suggests that the Ordovician and Silurian samples were deposited in lacustrine sulfate-poor or marine shale paleoenvironments (Figure 12). DBT/Phen results indicate a low sulfur concentration in the environment. The C29H/hopane values (Table 4, Figure 10(b)) also suggest that the deposition occurred in a marine shale environment. With all samples being characterized by low values or lack of gammacerane and absence of β-carotane, hypersaline conditions were not reported [114, 115].

The lack of okenane, isorenieratane, and aryl isoprenoids revealed the absence of green and purple sulfur bacteria from the photic zone, thereby implying the deposition in deeper basins. The absence of these bacteria indicates that the photic zone was more oxic than the zones at the sediment water interface. Photic zone anoxia, supported by the presence of diaryl isoprenoids, was observed only in samples from the Kętrzyn IG-1 borehole (Figure 11(a)).

All analyzed samples contain a wide distribution of polycyclic aromatic hydrocarbons (PAH), which, in this case, should be linked with thermal maturity. Although in some cases, such as the Kętrzyn IG-1 borehole, organic matter is immature or early mature, which likely implies that part of PAH presented in analyzed samples had nonpetrogenic source. Fl/(Fl+Py) values indicate mostly pyrogenic origin [116]; however, these data were not supported by other PAH diagnostic ratios like BaA/228. Benzo[a]anthracene (BaA) was detected in seven samples, and six of these belong to the Wenlock (Table 10). This information suggests that possible biodegradation processes could have affected Fl/(Fl+Py) values [117, 118]. Minor amounts of terrigenous input were detected from some samples in the Kętrzyn IG-1 borehole. The synthesis of information about PAH distribution (especially appearance of BaA) and terrigenous input suggests that some PAH in these samples might be evidence of limited Silurian wildfires observed in previous literature [119]. However, a lack of inertinite group macerals (Table 9) suggests absence of paleowildfire activity. The biomarkers which should be linked with bryophytes and fungi like retene and perylene have been observed only in the Kętrzyn IG-1 borehole, as the paleogeographical position of other boreholes was further away from the land massif [93]. Limited terrestrial organic matter input might be linked with higher concentrations of vitrinite-like particles (Table 9). However, some researchers suggest that retene also can be linked with, e.g., green algae [120]. This source of retene cannot be ignored especially in the case when samples from the Kętrzyn IG-1 borehole indicated significant green algae input.

Another association of PAH with origin of organic matter is benzo[e]pyrene, which is also a phytoplankton biomarker [121]. Largely, the phytoplankton origin of analyzed organic matter explains the high abundance of this compound.

Furthermore, the presence of 6-ring PAH and high values of the Fl/(Fl+Py) ratio suggest that these PAH marked in all samples might be linked to volcanism and/or hydrothermal activity as 6-ring PAHs were found in hydrothermal oils from the Guyamas Basin, exhibiting greater concentrations during significant volcanic activity [122, 123].

5.4. Thermal Maturity of Organic Matter

The values of Tmax measured in individual stratigraphic horizons do not show significant variability (Tables 1 and 2); that is, they do not increase with burial depth (Figure 13). The Sasino Fm. is an exception to this pattern; the maturity of organic matter increases with an increase in depth. The differences in the maturity of the organic matter were observed only spatially. Samples from the western part of this area (Hel-Gdańsk–Malbork–Prabuty zone) (Figure 1(b)) have slightly higher Tmax values (Figure 13), with a median of 430–440°C, whereas samples from the eastern part (Dębowiec Warmiński–Lidzbark Warmiński–Kętrzyn–Barciany zone) (Figure 1(b)) have lower Tmax values, <430°C (Tables 1 and 2). Thermal maturity classification by Peter and Casa (1994) [83] shows that the observed Tmax values do not seem to agree with VRo data; Tmax445°C is equal to 0.65% Ro [83]. Many studies have demonstrated that graptolite or solid bitumen could be robust thermal maturity indicators (review in [78, 107]). It is important to recalculate the reflectance of solid bitumen and graptolite reflectance values to the vitrinite equivalent. In this study, we obtained very similar thermal maturity based on recalculated reflectance of both solid bitumen and graptolite, when employed in formula proposed by Waliczek et al. [124] (VRo=0.0152×Tmax5.938).

The Tmax values imply that the organic matter reached a relatively low thermal maturity corresponding to the end of diagenesis and beginning of the “oil window” (Figures 8 and 13). Such a distribution of maturity corresponds with the regional trend of maturity changes from the edge of the TTZ toward the eastern periphery of the basin [125, 126].

The lowest thermal maturities expressed by organic matter reflectance were documented in the eastern part of the study area (Kętrzyn IG-1 borehole and one sample Olsztyn IG-2 2398.7). EqVRo for these samples are in the range of 0.53–0.66% (Table 8). Except for the three previously mentioned samples (two samples from the Kętrzyn IG-1 borehole and one sample from Olsztyn IG-2), recalculated maturity from solid bitumen (VRb), graptolite, chitinozoa, and vitrinite-like particles (VRg), in the analyzed rocks indicates organic matter maturity with hydrocarbon generation potential. The maturity levels differ depending on the empirical formula used. Samples with the highest organic matter reflectance values were obtained from the Pelplin (Prabuty IG-1 borehole) and Pasłęk (Gdańsk IG -1/3079.2; Gdańsk IG-1/3087.5) Formations. Thermal maturity in these samples is equivalent to the beginning of “gas window” according to the formulas of Landis and Castaño [75] and Luo et al. [78] or to the peak of “oil window” using the other equation given in Table 8. Recalculated vitrinite reflectance results confirm the regional trend of thermal maturity changes, with the mature western part and the immature eastern part. However, the eastern part is represented only by samples from the Kętrzyn IG-1 and Olsztyn IG-2 boreholes, which may result in an inaccurate maturity trend.

Generally, the results of the reflectance measurements of organic matter, dispersed in the analyzed Ordovician and Silurian samples, correspond with previous microscopic studies [127, 128]. However, in some samples (Prabuty IG-1/3290.5, Prabuty IG-1/3328, and Gdańsk IG-1/3096.2), the reported thermal maturity is higher than the current contribution, indicating the gas productive phase [127]. These inconsistencies may be due to methodological reasons. In previous studies, thermal maturity maps based on measured reflectance values were not recalculated to the vitrinite reflectance equivalent. Moreover, Caricchi et al. [129] reported the oil productive phase in the Lower Paleozoic, rather than gas productivity, using Fourier transform-infrared spectroscopy, X-ray diffraction on clays, and organic petrography. As reported in previous studies [127, 128], reflectance measurements typically represent the mean reflectance values for organic particles such as vitrinite-like particles, solid bitumen, or graptolite. As shown in Figure 4, the specific organic components have different reflectance values, and this phenomenon has already been discussed in many studies ([76, 78] and references therein). Typically, the graptolites exhibit a higher reflectance value than the solid bitumen ([78] and references therein) (Table 5). An appropriate recalculating formula should be selected to properly determine the thermal maturity of organic matter. The vitrinite reflectance equivalent (Table 8) shows differences depending on the application of the equations. Comparing recalculated measurements on solid bitumen and graptolite reveals that the Landis and Castaño [75] and Luo et al. [78] equations likely provide slightly overestimated values for the thermal maturity of organic matter. Moreover, the Tmax value from Skręt and Fabiańska [88] for Ordovician and Silurian rocks (Gdańsk IG-1, Hel IG-1, and Prabuty IG-1) when recalculated to VRo values using the Waliczek et al. [124] formula indicates almost identical thermal maturity obtained in this study using equations of Jacob [74], Petersen et al. [76], or Waliczek et al. [124] (Table 8).

Additional information about thermal maturity was provided by biomarker analysis. The preliminary thermal maturity of the organic matter was defined based on the presence/absence of PAH distribution. The presence of perylene indicated that the organic matter did not reach 0.7% on the Ro scale. Benzo[a]pyrene (BaP) disappeared when the thermal maturity of organic matter reached 0.9% on the Ro scale [130]. According to these data, samples from the Kętrzyn IG-1 borehole contained immature or early mature organic matter as they contain perylene (Table 10). Other samples did not contain BaP, which suggests that the maturity level of these samples is above 0.9% on the Ro scale. This is primarily the result of the location of the samples, most of them being from the western part of the study area, the Darżlubie-Gdański-Hel. The eastern part is represented only by Kętrzyn IG-1 and Olsztyn IG-2.

Thermal maturity sterane indicators C29 S/(S+R) and C29ββ/(αα+ββ) suggest that all samples, except those from the Kętrzyn IG-1 borehole, contained mature organic matter (Figure 14). The Kętrzyn IG-1 borehole contained early mature organic matter. The lower maturity of samples from Kętrzyn IG-1 was indicated by the C31 S/(S+R) ratio. Silurian samples from this borehole were characterized by considerably lower C31 S/(S+R) results (0.26–0.27) than the other samples that range between 0.56 and 0.65 (Table 5). The same situation was observed after the analysis of the Ts/(Ts+Tm) and Mor/Hop results. In both cases, results for Kętrzyn IG-1 suggest that this borehole contained more immature organic matter than other boreholes. This situation was also observed on the Shanmugam diagram (Figure 9). As CPI results are not supported by other biomarker indicators, those for Kętrzyn IG-1 seem to be affected by other sources or paleoenvironmental conditions (Table 5).

DNR-1 and TNR-1 based on alkylnaphthalenes provided similar results for all samples. However, the differences between the obtained DNR-1 results were higher, which might be linked to water-washing as alkylnaphthalenes are soluble in water and DMN should be washed before TMN [88, 131]. The vulnerability to water-washing is visible more in the case of DNR-1, which should increase with increasing thermal maturity [132]. In some boreholes (such as Darżlubie IG-1), the DNR-1 values did not increase with increasing thermal maturity, as suggested by other indicators or methods. It is likely that water-washing is responsible for this disturbance in DNR-1 results. Moreover DNR-1 results show low sensibility in samples with thermal maturity lower than 0.9% Ro [132]. Hence, the results of DNR-1 and TNR-1 should be analyzed very carefully and compared with other maturity results.

Most of the calculated RcalMPI1 values are reliable considering the above indirect thermal maturity biomarkers (Table 5). These results were unreliable in only three cases. In the Olsztyn IG-2/2398.7, sample results of RcalMPI1 are slightly underestimated, whereas they are overestimated in samples from the Kętrzyn IG-1 borehole. This is additional evidence that the organic matter in these samples did not reach 0.65% on the Ro scale as the MPI-1 indicator is not suitable for samples with maturity below 0.65% Ro [133]. All samples, except the Kętrzyn IG-1, contain mature organic matter. The Sasino Fm. sample from the Olsztyn IG-2 (2398.7 m [7870 ft]) and the Wenlock–Ludlow intervals of the Pelplin Fm. from the Darżlubie IG-1, Gdańsk IG-1, and Hel IG-1 boreholes mark the peak of the “oil window.” Other samples from the Darżlubie IG-1, Hel IG-1, and Prabuty IG-1 boreholes are at the boundary between the oil and gas windows. The Sasino and Pasłęk formations in the Gdańsk IG-1 borehole are in the “gas window.” Significant maturity changes between Pasłęk and Prabuty formations observed in RcalMPI1 results from the Gdańsk IG-1 borehole (Table 5) probably are caused by migration as thermal maturity indicators based on aliphatic fraction did not record a high increase in organic matter maturity [134]. The presence of fluorescing organic particles observed in reflected light during microscopic analysis indicates that the thermal maturity level is not higher than 1.30% Ro [105].

The presence of PAH might be evidence of hydrothermal activity, probably linked with volcanism [122, 123]. This theory is supported by the sudden increase in the thermal maturity of organic matter in the Gdańsk IG-1 borehole, where the difference in maturity between the Pasłęk and Pelplin formations is significant. The sudden increase in organic matter maturity in the Gdańsk IG-1 borehole might also be linked to the thermal influence of TTZ activity caused by the collision of Eastern Avalonia and Baltica during the Upper Ordovician–Lower Silurian periods [88].

In general, the indicators of thermal maturity of the organic matter used in this study for the Middle–Upper Ordovician and Lower Silurian, despite some differences resulting from the limitations of the used method, exhibit a clear trend. In the studied area of the Baltic Basin, the maturity of organic matter is in the oil window phase and perhaps even in the initial phase of the gas window in the western part of the basin and decreases to the east, reaching the immature phase in the Kętrzyn-Olsztyn area.

The petroleum system of the Early Paleozoic Baltic Syneclise comprises numerous organic-rich horizons and sandstone reservoirs. The Late Cambrian–Early Ordovician black bituminous shales are the most recognized organic-rich horizons. The presence of effective source rocks in these horizons is confirmed by numerous deposits of oil and gas, primarily located in the Middle Cambrian reservoir rock. Current research has shown that rocks meeting the source rock criteria are also present in the rest of the Early Paleozoic profile, in the Middle–Late Ordovician and Early Silurian rocks.

  • (1)

    In the Polish part of the Baltic Syneclise, the best source rock parameters and indices were observed in the Middle–Late Ordovician (Sasino and Prabuty) and the Early Silurian (Pasłęk and Pelplin) formations. In the eastern part of the Baltic Syneclise, their total thickness varies from a few hundreds of meters in the east to approximately 1,500 m in the west

  • (2)

    The maximum measured TOC content in these strata is 10.6 wt. % in the Sasino Fm., 12.9 wt. % in the Prabuty Fm., 14.6 wt. % in the Pasłęk Fm., and 3.1 wt. % in the Pelplin Fm., with median values of 1.96, 1.23, 0.91, and 1.15 wt. %, respectively

  • (3)

    The analyzed successions are dominated by immature/early mature, algal (oil-prone) type II, and mixed II/I kerogen deposited in a marine paleoenvironment with anoxic conditions at the bottom and oxic conditions in the photic zone

  • (4)

    Though the petroleum potential is good to very good, the organic matter maturity level is differential. Combined geochemical and petrographic results indicate that in the western part of study area (Darżlubie–Hel–Gdańsk area), organic matter is mature. Early-mature organic matter was noted in the central part of study area (Kętrzyn-Olsztyn area). The eastern part of study area (Gołdap area) contains immature organic matter. In general, organic matter maturity rises westwards. The western part of the study area alone exhibits a lower potential where the organic matter is in the “oil window” phase

  • (5)

    Cyanobacterial activity and oxygen-depleted conditions in the photic zone were observed from samples located near the land massif. The presence of dehydroabietic acid methyl ester in all samples and the lack of terrigenous input in the majority of them supports a cyanobacterial origin of the mentioned ester in the analyzed samples

  • (6)

    The presence of plant-derived aromatic biomarkers and elevated values of vitrinite-like particles in Silurian samples from the marginal zone of the basin might be interpreted as evidence of land plant input. PAHs have a petrogenic origin or are evidence of hydrothermal maturity linked to volcanism noted in the study area. PAH distribution could be affected by biodegradation processes


The authors declare that they have no conflicts of interest.

This study was financially supported by the National Science Centre (NCN, grant no. 2016/21/B/ST10/02079) and AGH University of Science and Technology (grant no. We extend our sincere appreciation to the staff of the AGH Department of Geology, Geophysics, and Environmental Protection Laboratory for their support in geochemical analyses.

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