Abstract The recognition of a forebulge in the subsurface is difficult because of its low amplitude and wide extent. It is further complicated by the subsequent tectonic modification (by the Laramide orogeny in this case) that may have overprinted the forebulge with complex younger structural patterns. Three-dimensional (3-D) flexural numerical modeling provides a strong supportive tool to help predict forebulge locations and focus subsurface search on their subtle isopach expression. Based on detailed well-log correlations and good outcrop control, three regional cross sections were established to identify Late Cretaceous forebulges in southwestern Wyoming. Along these sections in the Greater Green River Basin (two east-west and one northwest-southeast), the existence of forebulges was only recognized in the southern section. In response to the progressive eastward movement of the Crawford, early Absaroka, and late Absaroka thrusts, the forebulge migrated eastward to the Moxa Arch, the Rock Springs Uplift, and the Washakie Basin, respectively. The 3-D flexural modeling indicates that the forebulge was limited in its extent only to the southern part of the basin because of the distribution of thrust loads. The forebulge shifted southeastward over time because of the migration of these loads. The 3-D flexural modeling is critical to understanding Late Cretaceous forebulge migration across southwestern Wyoming.

Abstract This AAPG volume is the first to concentrate on the occurrence, distribution, and character of lacustrine sandstone reservoirs and to put those reservoirs into a hydrocarbon system and depositional process context. Although much research has been conducted on lacustrine systems, most of that work concentrated on reconstructing paleoenvironments, deciphering paleoclimate, or estimating hydrocarbon source potential. As of 2008, about 2.2% (16 × 10 9 bbl; 2.5 × 10 9 m 3 ) of the world's discovered oil has hydrocarbon sources associated with lacustrine or coal-bearing strata, whereas lacustrine reservoirs appear to account for only about 7% of daily oil production (~5 × 10 6 bbl of oil/0.8 × 10 6 m 3 ). Lacustrine reservoirs appear to hold only about 3% of proven oil reserves (based on U.S. Geological Survey and BP published information; see chapter by Bohacs in this volume for more details). Does this small proportion discovered in lacustrine reservoirs reflect a fundamental problem with lacustrine reservoirs or a great opportunity for exploration? This is a major question addressed in this volume. The answer may be that lacustrine reservoirs pose both great opportunities and great challenges. Lakes are complex dynamic systems whose behavior can differ significantly from marine systems. Predictions of hydrocarbon reservoir presence, distribution, and character in lake systems similarly pose distinct challenges. These challenges arise from the fundamental nature of lacustrine systems: nonunique relations of lake character to climate or tectonics, contingent responses of lakes to climate change, and variable ties among lake level, sediment supply, and water supply. At the hydrocarbon-reservoir scale, these challenges affect

Abstract A relatively small range of lacustrine-facies associations record the complexly contingent interactions of a wide range of physical, chemical, and biological processes (climate, tectonics, sediment supply, vegetation, landscape evolution). Each lacustrine-facies association contains fluvial, lake-plain, lake margin, and lake center strata with characteristic hydrocarbon reservoir potential. The accumulation of these lacustrine-facies associations and their potential hydrocarbon reservoirs arise from interactions of typical ranges of rates of potential accommodation and sediment plus water supply and can be interpreted genetically as overfilled, balanced-filled, and underfilled lake-basin types. Fluvial-lacustrine lacustrine-facies associations (interpreted as forming in overfilled lake basins) generally contain reservoirs that are best developed in aggradationally stacked high-stand clastic shoreline strata and occasionally in skeletal carbonate or charophytic algal litho-somes or in lowstand incised valley fills and lake floor fans (basinally restricted turbidite and mass flow deposits). These reservoirs tend to have low vertical permeability (Kv) because flooding surfaces are generally marked by decreased input of coarse sediment and increased subsidence. They have the lowest average net reservoir:gross interval of the three lacustrine-facies associations. They do, however, have the highest average porosity and permeability and contain the largest overall reserves, mainly in lake-plain fluvial strata. Fluctuating profundal lacustrine-facies association (balanced-filled lake basins) have reservoir facies that include lake floor fans, incised valley fills, and shoreline clastics or carbonates deposited during transgressions and highstands. These reservoirs tend to have the smallest lateral extent and lowest average recovery factor of the three lacustrine-facies associations; (based on reservoir and fluid properties), but do have good vertical and horizontal permeability (Kh) in highstand and transgressive systems tracts and the best Kv of all the lake-basin types. Evaporative lacustrine-facies associations (underfilled lake basins) contain reservoir facies that are best developed in transgressive sheetflood clastics, early highstand fluvial channels, and late highstand shoreline carbonate grainstones. Early carbonate and evaporite cements are common in these reservoirs, and there tends to be a wide lateral displacement of high-stand from lowstand systems tracts. They do, however, have the best Kh (because of common erosion that enhances lateral connectivity) as well as the thickest net pay of all the lake-basin-type reservoirs (as they tend to occur at relatively large potential accommodation rates). Associated fluvial styles among the lacustrine-facies associations (lake-basin types) appear to vary systematically, as a function of sediment plus water supply relative to potential accommodation rates: perennial, high sinuosity streams are most common in overfilled lake basins, intermittent to perennial low-sinuosity streams in balanced fill, and a wide range from ephemeral sheetflood or multithread braided streams to perennial high-sinuosity streams in underfilled lake basins. Observations indicate that these associations of hydrocarbon reservoir and seal play elements occur in a wide variety of tectonic settings and ages, from continental rift to convergent foreland basins of the Cambrian to Holocene. Continued success in economic discovery and efficient recovery depend upon continued testing and elaboration of these concepts and a deeper understanding of the essential processes controlling deposition of lacustrine strata.

Abstract A long history of petroleum production from the South Caspian Basin demonstrates this region's significant hydrocarbon resources. The world's first oil wells were located here in the early 19th century, and mechanized drilling led to increasingly advanced extraction programs from the mid-1800s into the Soviet era. Offshore areas, first explored in the 1930s, remain the object of significant interest and potential. Hydrocarbon reservoirs belong to the Productive Series (Miocene-Pliocene), a fluviolacustrine complex of sediment delivered by the paleo-Volga river system. Extremely high subsidence rates and closure of the lake basin contributed to a distinctive stratigraphy with basinwide layer continuity. A combination of oil-prone to mixed gas/oil basinal marine source rocks of Oligocene to Miocene sourced the petroleum. Hydrocarbon generation started in the basin about 4 Ma, with most generation and expulsion occurring in the last 2 m.y., the same time structures began to form in the basin. High fluid pressure gradients appear to inhibit the development of effective traps with seals, particularly offshore Azerbaijan where the occurrence of shale exceeds sand.

Abstract The Pliocene Productive Series of the South Caspian Basin records a major lowstand relative to pre-existing Miocene and subsequent Pleistocene and Holocene shelf margins. The Productive Series records approximate 2.6 m.y. of relatively continuous deposition of lacustrine sediments and reaches more than 6 km (0.62 mi) in thickness. Regional-scale seismic mapping has allowed an interpretation of the basin evolution during the deposition of the Productive Series. The Productive Series is subdivided into four phases of deposition. Differences between these phases are interpreted to reflect changes in the balance between sediment and water input and the evolving basin morphology. Basin morphology inherited from a previous depositional phase strongly influence successive phases of deposition. Phase 1. Messinian: Base-level fall, roughly coincident with the Messinian Salinity Crisis, resulted in isolation of the Caspian Sea from the global oceans. Reconstruction of the basin profile immediately after this event indicates that the South Caspian Basins base-level fell by approximately 1.5 km (0.93 mi). Phase 2. Integration of drainage systems to this dramatically lowered base level resulted in the delivery of large volumes of sediment and water from the Russian Platform, Caucasus mountains, and Kopet-Dagh mountains to the South Caspian Basin through the Paleo Volga, Kura, and Amu darya/Uzboy rivers. Within the South Caspian Basin, fluvial and fluvio-lacustrine facies were deposited in preexisting structural depressions, forming the Lower Productive Series. Phase 3. Infill of paleotopographic depressions by the Lower Productive Series resulted in the formation of a low gradient ramp over much of the South Caspian Basin. Middle Productive Series strata were deposited on this low-gradient ramp as aerially extensive fluvial, deltaic, and lacustrine facies. Reconstructed basin profiles show a reduction of the depositional gradient as the result of sediment infill. Phase 4. During deposition of the Upper Productive Series, the ratio of sediment supply to water supply decreased as much of the coarse-grained sediment supplied from the Paleo-Volga was deposited updip. Thus, the updip depositional profile became increasingly low relief. This change led to the creation of a relatively deep-water lake in the center of the South Caspian Basin surrounded by lacustrine shelf margins. Evaporites were deposited in the center of this lake, suggesting that it was subject to episodic phases of desiccation. Overlying the Productive Series is a regionally extensive marine condensed section, the Akchagyl Suite. The Akchagyl Suite records a major regional transgression and a return to marine conditions as the South Caspian Basin was reconnected to the global oceans. Pleis-tocene shelf margin complexes downlap onto the Akchagyl Suite and record progradation of shorelines into a deep-water brackish lake (~1000 m [3280 ft]). This Pleistocene depositional system was similar to the present-day depositional setting of the Caspian Sea.

Abstract This chapter presents a comprehensive analysis of the nature and origin of the observed cyclic patterns in the late Miocene and Pliocene rocks that are the main hydrocarbon reservoirs along the Azerbaijan margin of the Caspian Sea. Data from extensive onshore studies of outcrop sed-imentology were combined with well-log interpretations from the offshore Azeri-Chirag-Deep Water Guneshli oil field to quantify the nested nature of depositional cycles inferred to represent 20 ka, 100 ka, and 400 ka astronomically driven climate cycles. Spectral analysis of gamma-ray logs supports the conclusion that much of the Productive Series in the South Caspian Basin (base KS to top of the Balakhany) records the controlling influence of astronomical changes in insolation that acted in phase across both the Volga drainage basin and the Caspian Sea. We constructed an idealized depositional sequence and its link to the lake level/climate drivers based on evidence for cyclic sedimentation from spectral analysis of the gamma-ray logs, the range of depositional systems interpreted, and the climate signal derived from palynology. These sequences are expressed consistently in strata deposited in fluvial, lake-margin mudflats, shoreline, and lake center settings. In the sandier stratigraphic intervals, the 20 k.y. sequence is expressed as follows. The sequence boundary is an exposure surface within mud-stones. Overlying the sequence boundary is generally a forestepping succession of terminal splay sandstones and mudstones suggesting slowly rising lake level. Above this, a stack of braided stream deposits is present that generally represents the dominant sandstone interval of the entire sequence. We interpret this as a lowstand systems tract (LST). The LST is abruptly truncated by a lacustrine flooding surface, which in turn is capped by a back stepping succession of more terminal splay deposits and density underflow strata. In most sequences, no definite expression of the highstand and falling stage systems tracts exists. This contrasts greatly with shallow marine depositional sequences, where the falling stage systems tract generally contains the best-developed sandstones. The recognition that climate drivers of astronomical origin did fundamentally control sedimentation in the Caspian Sea profoundly affects both petroleum systems modeling and reservoir modeling by reducing the degrees of uncertainty compared with what is commonly the case in other less ordered depositional systems.

Abstract The Caspian Basin is one of the largest and most attractive petroleum basins in the world, even after more than 100 yr of production. Numerous rich oil and gas fields in the Caspian are present, and the potential for further exploration and development exists. However, the petroleum system in the northern part of the Apsheron anticlinorium is poorly understood. This study focuses on South Caspian reservoir components of the petroleum systems in the northern part of the postdepositional Apsheron Ridge. Two sets of three-dimensional (3-D) seismic data, acquired in the 1990s, were provided for study. The research focuses on the seismic expression of the Productive Series deposited in a lake or lacustrine environment. It was determined that tectonic control on sedimentation during deposition of the Middle Productive Series is the principal control on reservoir development. The documentation of relative lake level change is important because it impacts reservoir facies architecture and potential reservoir quality in the study area. It was further concluded that in some locations, seals may have been breached by reactivation of basement faults. Seismic attributes including amplitude and spectral decomposition were applied to the 3-D seismic data for interpretation. Valleys and channels that have been interpreted through these seismic attributes may represent feeder systems to downslope depositional lobes with better down-structure reservoir quality. High-amplitude anomalies off the structures are indicative of these depositional lobes. Lobes that are faulted and isolated in the flank areas of the structures may exhibit better seal capacity.

Abstract The Azeri, Chirag, and Gunashli (ACG) fields, located offshore Baku in the Caspian Sea, are being developed under a 30 yr Production Sharing Agreement. Contiguous with the ACG accumulation, the shallow-water Gunashli (SWG) field has produced in excess of 800 MMSTB since 1980 from more than 220 wells. SWG provides an opportunity to study the static and dynamic behavior of reservoirs important to the development of ACG. BP, operator of ACG, contracted with the Reservoir Modeling Centre of the State Oil Company of Azerbaijan to perform a study of the Balakhany IX and Balakhany X reservoirs in the SWG development. This study comprised construction of geologic descriptions for both reservoirs and their use in dynamic simulation models. This was achieved using hand-contoured geologic mapping, incorporating depositional trends such as channel architecture. Matching historical performance data from these reservoirs verified the underlying geologic models and provided confidence in predictions derived from them. Results of this study show that channel width was the key to accurate estimation of stock tank oil initially in place, and that to achieve this, as many wells as possible must be incorporated. In addition, vertical communication between sublayers of the reservoir was defined using geologically derived maps referred to as “interface transmissibility.” Thus, a set of deterministic maps based on well data provided an approximate three-dimensional (3-D) representation of the Balakhany IX and Balakhany X stacked channel architecture. This work will assist in the planning and optimization for the development of the ACG field.

Abstract This study presents facies descriptions and interpretations of the Eocene Sunnyside delta interval of the Green River Formation in Nine Mile Canyon, Utah, to document and constrain reservoir architecture and heterogeneity in lacustrine strata. Detailed measurement of vertical outcrop sections, paleocurrent directions, lithologic descriptions, and facies mapping of photopanoramas and digital Light Detection and Ranging (LIDAR) data were completed to define and interpret stratigraphic architecture at centimeter to meter scales. Results favor a depositional setting that fluctuated among delta plain (overbank, distributary channel, and mouth bar), marginal lacustrine, shoal water, and offshore lacustrine deposits. These strata were deposited at a distributary-dominated shoal-water deltaic margin and form four pro-grading parasequences that coarsen upward over the approximately 150 m (492 ft) thick study package. Measured channel dimensions and calculated sinuosities indicate a meandering fluvial system, and channel deposits include isolated lenticular, amalgamated lenticular, and amalgamated undulatory sandstones. Along with distributary mouth bar deposits, these channelized sandstone bodies represent the most promising reservoir units based on their lateral continuity and stacking geometries (ranging from ~40- to 115-m [~130- to 377-ft] wide × 4- to 6.5-m [13- to 20-ft] thick), as well as measured porosityand permeability relationships. Quantified dimensions of sandstone bodies and characterization of their lateral and vertical connectivity are significantly aided by a digital outcrop model. This model was generated by ground-based LIDAR, and results were exported as spatial and geologic constraints for geocellular modeling. Such quantitative outcrop-based modeling provides guidelines for predicting sub-surface reservoir quality and dimensions,and may be extrapolated to analogous fluviodeltaic and lacustrine hydrocarbon prospects.

Abstract This chapter presents three groups of reservoir simulation models that address aspects of fluid flow in deltaic systems of lacustrine basins. These models are based on companion field-based geologic investigations of the Eocene Green River Formation in Nine Mile Canyon of the Uinta Basin, Utah ( Taylor and Ritts, 2004 ; Moore et al., 2012 ). The first group consists of models based on two-dimensional (2-D) geologic descriptions of outcrops in Parley Canyon ( Taylor and Ritts, 2004 ). These descriptions, combined with petrophysical data from measured sections to produce a three-dimensional (3-D) model based on one-dimensional (1-D) and 2-D data. Because results of the first simulation models demonstrated the importance of channels in the plumbing of such lacustrine deltaic systems, a second group of simulation models was constructed to address the impact of heterogeneity within individual channels on flow across and along channels. The data used to generate these channel models are entirely synthetic, although the geometries involved were digitized from sketches drawn by a geologist and guided by relationships observed in the Uinta Basin study area. The third group of models represents the northwest Argyle Canyon area and, unlike the Parley Canyon model, was constructed from a third geologic data set that was produced by integrating light detection and ranging (LIDAR) scanning and correlation with geologic description of multiple cliff outcrops of varying orientation in the field ( Moore et al., 2012 ). Together, these models demonstrate dominant characteristics that control production efficiency of reservoirs in lacustrine deltaic settings and also demonstrate how improving geologic characterization improves the 3-D modeling of such systems.

Abstract The Green River Formation of Wyoming represents a complex sequence of lacustrine, fluvial, and playa strata deposited in and associated with Eocene Lake Gosiute. Detailed mineralogical and organic analysis of 10 cores from the Green River Basin identified correlatable stratigraphic intervals that can be used to interpret the geochemical and sedimentologic history of Lake Gosiute. More than 30 minerals were identified in this study, with graphic correlations and geostatistical evaluations defining 16 major minerals and oil yields, as defined by modified Fischer assay. These relations could be used to define distinct correlatable stratigraphic intervals that, when used in conjunction with known chronostratigraphic volcaniclastic units, could be used to interpret major influences upon the depositional history of Lake Gosiute. The four principal influences upon mineralogical stratigraphy of Lake Gosiute were determined to be (1) deposition and preservation of organic matter, (2) volcaniclastics, (3) periods of evaporation and concentration, and (4) influxes of fresh water accompanied by increased transported detrital material. Climatic variation both on broad and seasonal scales is interpreted to have influenced the mineral suite. The changes among these members have been alternatively interpreted as recording changes in lake-water hydrology, not necessarily related to climate changes ( Carroll and Bohacs, 1999 ; Pietras et al., 2003 ). Wet periods in the history of Lake Gosiute (Tipton and Laney members) were associated with increased organic production and a related mineral suite, whereas dry periods (Wilkins Peak Member) resulted in decreased organic production and the genesis of a prolific suite of saline and evaporite minerals. This systematic investigation of the distribution, chemical composition, and structure of the minerals in these sediments supported a playa-lake depositional model for the members covered by this study for genesis of the Green River Formation inLake Gosiute.

Abstract The late Miocene-early Pliocene Lake Pannon was a large, long-lived, brackish lake that occupied the Pannonian Basin system in Central Europe. Traditionally, the stratigraphic subdivision of its several-kilometer-thick sediment pile has been based on fossils of endemic mollusks. For a long time, however, stratigraphers were misled by the unconventional architecture of the lacustrine-deltaic sequence. Due to progradation, the increasingly younger sediment packages have been deposited horizontally next to each other instead of forming a purely vertical succession. This pattern was fully recognized recently by seismic exploration of the basin. Newly interpreted biostratigraphy using anagenetically evolving mollusk lineages (in the littoral and sublittoral facies) and dinoflagellate algae (in the sublittoral and profundal facies) is consistent with this progradation model. The regional Lake Pannon stratigraphy is tentatively correlated with the geologic time scale through mammal stratigraphy, magnetostratigraphy, and radio-metric age measurements. This correlation suggests that temporal resolution of the biozones is on the order of 1 m.y.

Abstract Doba Basin, Chad, is situated within the Central African rift system and contains up to 13 km (8 mi) of Cretaceous alluvial and lacustrine deposits. The complex climatic and tectonic evolution of the region, spanning the early to late Cretaceous is uniquely recorded in the stratigraphic architecture. In this chapter, we present a model of alluvial and lacustrine fill, which invokes high-frequency climatic fluctuations superimposed on longer term climatic cycles and variable accommodation produced by extensional tectonic processes. Upper Cretaceous (Cenomanian-Coniacian) Doba Basin stratal successions are interpreted to represent alluvial depositional systems that systematically varied along their longitudinal depositional profile, ultimately terminating in a lake, analogous to modern Lake Chad. Three distinct facies belts characterize the alluvial succession and one facies belt describes the lacustrine succession. Structurally controlled, sediment entry points fed upstream (updip) alluvial facies are composed of amalgamated midchannel bars that developed within multiple coeval low-sinuosity channels. These channels are stacked vertically and laterally, forming thick and aerially extensive channel complexes. Medial (middip) alluvial facies consist of semiamal-gamated bars. Channel complexes in medial depositional regions are thinner and less persistent than upstream channel complexes. Distal (downdip) fluvial-facies belts are characterized by a more heterolithic style of sedimentary bedding, suggesting that deposition occurred during fluctuating slack water and relatively low-energy discharge conditions. Distal channel complexes are thin and more laterally extensive than those of the medial regions. The fluvial system terminates in shallow ephemeral to perennial ponds and lakes, building small coalesced terminal splay complexes. Channel complexes that accumulated within the three alluvial facies belts are overlain by mudstone-prone flood-plain deposits, both of which combine to form alluvial depositional sequences. Amalgamated channel complex elements constitute the sand-prone reservoir intervals and consist of vertically and laterally stacked channel-fill elements. Channel complex elements are bounded below by regional unconformities and above by abandonment surfaces. Amalgamated channel complexes are overlain by a mudstone-prone interval dominated by flood-plain deposits containing isolated channels and are referred to as nonamalgamated channel complex elements. The mudstone-prone intervals are bounded at their top by regional unconformities. Age constraints for alluvial depositional sequences indicate that they accumulate during time spans of 200 to 500 k.y. Depositional sequences cluster vertically and laterally to form an alluvial sequence-set element. Three types of sequence-set elements are defined and include amalgamated, semiamalgamated, and nonamalgamated sequence sets. Sequence-set elements form the building blocks of an alluvial composite sequence, which is bounded above and below by regional unconformities. Composite sequences in the Upper Cretaceous strata of Doba Basin, Chad, range from 1 to 3 m.y. The alluvial and lacustrine depositional facies architecture and sequence-stratigraphic framework of the upper Cretaceous strata, Doba Basin, Chad, are attributed to high-frequency climatic fluctuations of 200 to 500 k.y. duration superimposed on longer term climatic cycles of 1 to 3 m.y. and variable accommodation that occurred during late-stage extensional processes. The stratal architectures of depositional sequences and composite sequences are interpreted to be controlled by an extrinsic forcing mechanism of climatic cycles that influenced water discharge and sediment transport. Humid periods correspond to times of erosion, transport, and deposition of coarse clastic sediment. These stratigraphic intervals are dominated by fluvial confined-flow sandstone elements and are commonly bounded above and below by unconfined-flow mudstone facies that also formed during humid climatic conditions. Arid periods correlate to intervals of low coarse clastic supply to the basin and widespread deposition of flood-plain unconfined-flow mudstone deposits. The cyclic variations of discharge and sediment flux that occur at 200 to 500 k.y. correspond to the accumulation of alluvial depositional sequences and those of the 1- to 3-m.y. cycle relate to the deposition of alluvial composite sequences. Understanding the complex interplay of the long-term structural evolution of the Doba Basin and climatic fluctuations recorded within the Cretaceous strata is essential in the accurate prediction of reservoir and seal distribution and continuity. This understanding has proven critical in developing an effective depletion strategy for the three field areas of southern Chad.

Abstract Modern lake basins set within active continental rifts provide useful analogs for exploration efforts in ancient extensional basins that are known to be rich in hydrocarbons. Lake Albert is one of the Great Lakes of Africa and is located at the northern end of the western branch of the East African rift system. This large, but comparatively shallow, eutrophic, and probably geologically ephemeral lake basin serves as an end-member example of the modern tropical lake systems that occupy this extensional province. Seismic reflection and gravity data sets indicate that the basin contains a maximum of 5 km (3.1 mi) of synrift, dominantly lacustrine sedimentary fill, in two subbasins separated by a midbasin high. In contrast to other large rift basins in the western branch of the rift valley, the Lake Albert Rift is not a highly asymmetrical half-graben basin, but instead has subsided nearly symmetrically and continuously in the late Cenozoic along two extensive boundary fault systems on either side of the basin. Seismic sequences from across the basin were correlated to borehole stratigraphy from a deep well drilled on the Ugandan margin. These observations suggest that the basin has experienced a long-term change from a continuously open lacustrine, possibly deep lake system in the Miocene or early Pliocene, to an alternating shallow lacustrine and fluvial system in the mid and late Pleistocene. This history of basin evolution has led to the development of a rich hydrocarbon system.

Abstract In 2001, when appraisal and development work began at Daan field, the MI Energy-PetroChina partnership was faced with the commercial necessity to optimize production. Given the relatively low oil prices that prevailed then, the Daan field project was considered marginally commercial. It was imperative to define a cost-effective and technologically strong field development program. The main geologic risk factor was reservoir sandstone distribution, and a new model was constructed to predict distribution of the primary reservoir objective FuYang sandstones. Consistent with the structural evolution of the Daan area, the FuYang reservoir is focused on a series of north-northeast to northeast-trending depocenters controlled by down-to-the-northwest normal faults parallel and genetically related to the Daan fault system. The Fuyang reservoir sandstones and siltstones are interpreted to have been deposited in fluvial and marginal lacustrine environments associated with small- to medium-size lake systems, which developed in the north-northeast to northeast-trending depocenters. Based on core studies and observed modern analogs, the main FuYang reservoir facies comprised lake-delta mouth bar and fluvial channel-bar deposits. The FuYang reservoir is best developed in the north-northeast to northeast-trending sweet spots, which correspond to the structurally controlled depocenters. Although the FuYang is also oil productive at Daan outside these sweet spot areas, well productivity is lower because of reduced reservoir sandstone development. The development plan for the Daan field called for wells to be drilled in a consistent grid pattern (175 × 350 m [574 × 1148 ft]), but the order of drilling has a significant impact on the commerciality of the project, and the reservoir distribution model is key to defining the drilling order. Appraisal wells are drilled continuously in an effort to further refine the reservoir model and identify FuYang sweet spot areas. At the same time, development well drilling proceeds in priority order, focusing first on the best quality FuYang sweet spot areas. This systematic drilling approach has lowered reservoir risk and improved early well productivity by as much as 40 to 50%, thereby significantly enhancing the project’s profitability. Understanding of the FuYang reservoir distribution has also contributed to a large increase in the estimate of oil in place for the Daan field.

Abstract The Reco^ncavo Basin in northeastern Brazil records more than 20,000 ft (>7000 m) of clastic sediments deposited in eolian, shallow and deep lacustrine, deltaic, and fluvial environments during the Early Cretaceous rifting process, at the onset of the Gondwanaland breakup. During rifting, basin fill was controlled by the combined effects of tectonic activity and high-frequency climatically driven changes in lake level. Different sedimentation styles developed during the Neocomian as a function of the intensity of tectonic activity. During the phase of more intense tectonic activity, the basin was a half graben with a shelf area separated from a deep lacustrine depocenter by a narrow fault-controlled steep slope. During this stage, two main depositional systems developed in the half graben: (1) alluvial fans and deep-water sediment gravity flows from the rift’s eastern border fault and (2) immature fluviodeltaic systems in the half-graben shelf area (flexural zone) with associated deep-water prodeltaic gravity flows on the down-thrown block of the fault-controlled slope zone. The rapidly subsiding depocenter and fault activity during this period led to a sedimentary record characterized by a fining-upward stacking on the slope and depocenter and erosion or bypass in large areas of the half-graben shelf. The sedimentary strata related to this stage are herein called slope-controlled deep-water systems (SC-SS) fed by a fluviodeltaic system. With the decrease in fault activity, gradually, the feeder systems moved from the flexural zone and an axial fluviodeltaic system developed from the north, with deep-water aggradational sedimentation followed by shallow-water aggradational-to-progradational amalgamated distributary mouth bars dominated by interpreted hyperpycnal flows. The shallow lacustrine environment paired with a high sediment load caused rapid progradation during this stage. The strata related to this stage are herein called the axial deep- to shallow-water systems (AX-SS). Interaction between climate and tectonics controlled the rate of accommodation creation, sedimentation rate and sourcing in the area, forming the basis for a sequence-stratigraphic interpretation. A composite sequence boundary is interpreted at the base of the deep-water sediments in the SC-SS with a lowstand sequence set extending from local units B to D. Sedimentation at the axial area of the half graben (AX-SS) started later, corresponding to the unit C in the SC-SS. A composite transgressive surface is interpreted at the base of unit E, marking a long-term trend of decreasing sedimentation energy (transgressive sequence set), culminating in a region abandonment surface interpreted as a composite maximum flooding surface in unit G. Highstand sequence set is represented mostly by the aggrading and prograding deltaic sequences that correspond for the major part to the Marfim Formation. Another composite sequence boundary caps this composite sequence, corresponding to the local marker 15 at the top of the Marfim Formation.

Abstract In the Saar-Nahe Basin, at the turn of Carboniferous to Permian, lacustrine organic matter-rich mudstone (paper shales), algal limestone (biostromes and bioherms), and deltaic sandstone accumulated in lakes, ponds, and flood basins under a tropical climate. Lakes were probably interconnected frequently by lowland meandering rivers to form more or less continuous waterways with rich plant and animal life. The paper shales provide key insights into paleoclimatic conditions and their relation to sediment supply in this fluvial-lacustrine system. Lamination was formed by varying input of suspended sediment to the lakes of equatorial Central Europe. During the wet season (summer), silt-rich suspensions were swept out into the lakes, driven by heavy rainfall after depositing coarser grained sediment at the lake margin. During the dry season (winter), floods were infrequent, and siliciclastic suspension input to the lakes was minimal. During the entire year, the biological production that formed organic matter-rich dark clay layers of the laminites was more or less constant. This seasonality of clastic input resulted in a variable thickness of intervals of delicate light/dark laminite couplets that are attributed to storm or flood events (weather-bedded). These strata demonstrate that hydrocarbon reservoir and source-prone lithofacies can accumulate in relatively close proximity. In this system, shales with significant amounts of organic matter are genetically related to profundal limestones as well as to algal bioherms and biostromes forming in the same lake or pond. Interbedded organic matter-lean siltstone resulted from the same floods that carried coarse clastics from rivers into flood-plain ponds and lakes, forming lacustrine deltas. The fine-grained strata can be quite useful for understanding the distribution of hydrocarbon play elements as they contain a relatively complete record of depositional conditions that is integrated across the entire alluvial landscape.

Abstract Lake Michigan is the world’s sixth largest freshwater lake and has many features in common with oceanic settings, albeit at a smaller scale. All of the constructional features typical of ocean coasts can be found along the shore of Lake Michigan, and it has a shelf-slope system where coastwise rectification of currents, coastal downwelling jets, Coriolis veering of lake currents, benthic nepheloid layer, and density currents have been observed. Unlike ocean coasts, however, the wave climate is predominantly mild, and only a very small lunar tide exists, although other (quasi) periodic water-level fluctuations such as seiches and edge waves do occur. Another significant difference is the occurrence of quasicyclical climatically induced lake-level fluctuations of as much as 2 m (6.6 ft) that greatly influence the way that coastal sediments accumulate. Lastly, the Lake Michigan coast during the late Wisconsin and Holocene experienced multiple noncyclic transgressive and regressive events. Lake levels have been as much as 18 m (60 ft) higher and 60 m (200 ft), or more, lower than present, and changes have commonly occurred at rates several magnitudes greater than the most rapid eustatic sea level changes. In this chapter, we will show how hydrodynamic processes, cyclic and noncyclic lake-level changes, and the way in which sediments are supplied to the lake have interacted to shape the architecture of sedimentary deposits along the coast and in the deep basins. We will summarize the results of our own work, but we are also indebted to many researchers whose work is included in this narrative.