We investigated pinnacle features at the base of late Oligocene–Miocene isolated carbonate buildups using three-dimensional seismic and borehole data from the Browse Basin, Northwest Australia. Brightened seismic reflections, dim spots, and other evidence of fluid accumulation occur below most pinnacle features. An important observation is that all pinnacles generated topography on successive late Oligocene–Miocene paleo-seafloors, therefore forming preferential zones for the settlement of reef-building organisms by raising the paleo-seafloor into the photic zone. Their height ranges from 31 m to 174 m, for a volume varying from 33 km3 to 11,105 km3. Most of the pinnacles, however, are less than 2000 km3 in volume and present heights of 61–80 m. As a result of this work, pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates, and they are considered as precluding the growth of the larger isolated carbonate buildups. We postulate that pinnacle features above fluid-flow conduits demonstrate a valid seep-reef relationship, and we propose them to be refined diagnostic features for understanding fluid flow through geological time.
Carbonate strata dominate the Miocene stratigraphy of the Browse Basin, Northwest Australia (Rosleff-Soerensen et al., 2012), and deposition on other equatorial margins (Brouwer and Schwander, 1987; Eberli and Ginsburg, 1987; Grötsch and Mercadier, 1999; Wilson et al., 2000; Pomar, 2001; Fournier et al., 2005). Significantly, the Browse Basin records a change from a carbonate ramp to a rimmed platform during the Cenozoic, with regional data documenting the contiguous growth of carbonate buildups in the shallowest parts of the North West Shelf after the early Oligocene (Rosleff-Soerensen et al., 2012; Belde et al., 2017; Rankey, 2017). To understand what controls carbonate growth on equatorial margins, it is important to explain the onset of isolated carbonate buildup growth in the Browse Basin, and in similar carbonate sequences in the Browse Basin.
Locally, Howarth and Alves (2016) mapped clustered fluid-flow features above late Oligocene–Miocene karst systems and within isolated carbonate buildups. These fluid-flow features were interpreted as being associated with a salt diapir at depth, despite: (1) the absence of thick evaporites in great parts of the Browse Basin, and (2) the presence of the shallow-water Seringapatam Reef in the area interpreted by Howarth and Alves (2016). While the latter authors discussed fluid flow within (and above) the larger isolated carbonate buildups, they did not address the potential significance of migrating fluids on isolated carbonate buildup initiation and growth. Hence, there is still no complete understanding of what controlled the distribution of isolated carbonate buildups in the Browse Basin, and the role migrating fluids may have played during buildup evolution.
In this paper, we document for the first time the relationship between fluid flow and the growth of late Oligocene–Miocene isolated carbonate buildups in the Browse Basin through the interpretation of the Poseidon three-dimensional (3-D) seismic volume. This was done regardless of the presence of the Seringapatam Reef in the study area, as it forms a prominent near-surface carbonate platform that is distinct from the pinnacle features documented in this paper. Therefore, we addressed the following research questions:
(1) What is the origin of fluids and what structures controlled fluid flow in the Browse Basin?
(2) What do pinnacle features observed at the base of isolated carbonate buildups reflect in terms of geological processes?
(3) Was there an active seep-reef relationship in the Browse Basin during the late Oligocene and Miocene?
Mesozoic–Cenozoic Evolution of the Browse Basin
The Browse Basin is an offshore sedimentary basin on Australia’s North West Shelf, a northeast-striking passive continental margin developed from the Late Jurassic to the present day (Fig. 1; Stephenson and Cadman, 1994; Rosleff-Soerensen et al., 2012, 2016). Located on the southeast edge of the Timor Sea, the Browse Basin exhibits a margin-parallel, landward-dipping half-graben geometry (Fig. 2; Struckmeyer et al., 1998; Rosleff-Soerensen et al., 2012).
Jurassic continental rifting between Greater India and Western Australia (Veevers and Cotteril, 1978; Langhi and Borel, 2008; Rosleff -Soerensen et al., 2012, 2016) generated northeast-trending structures on the North West Shelf, including in the Yampi, Leveque, Caswell, and Barcoo subbasins (Fig. 1; Longley et al., 2002; Langhi and Borel, 2008; Rosleff-Soerensen et al., 2012, 2016; Howarth and Alves, 2016). Early Cretaceous subsidence and the deposition of a passive-margin sequence buried the rift-related topography (Fig. 2; Stephenson and Cadman, 1994; Struckmeyer et al., 1998; Langhi and Borel, 2008; Rosleff-Soerensen et al., 2012).
Seafloor spreading in the Indian and Southern Oceans caused northward migration of Australia from ∼40°S in the Eocene–Oligocene to ∼20°S at present (Apthorpe, 1988; McGowran et al., 2004; Rosleff-Soerensen et al., 2012). In particular, late Oligocene–early Miocene collision between the Pacific and Australasian plates promoted counterclockwise rotation of the Australian continent (Veevers and Powell, 1984). This rotation generated subsidence and shallow extensional faulting in the outer North West Shelf (Stephenson and Cadman, 1994). Fault reactivation (Harrowfield and Keep, 2005; Rosleff-Soerensen et al., 2012) and moderate tectonic inversion in the Browse Basin (Keep et al., 2000).
The Cenozoic oceanography of the North West Shelf was closely controlled by the Indonesian Throughflow (Fig. 1; McGowran et al., 1997; Gallagher et al., 2009; Kuhnt et al., 2004; Rosleff-Soerensen et al., 2012). The Indonesian Throughflow flooded the North West Shelf with warm, low-salinity water, delivering Pacific and Asian reef species (Collins, 2010; Rosleff-Soerensen et al., 2012). Bathymetric controls on the Indonesian Seaway during Miocene convergence between Australia and the Banda arc (McGowran et al., 1997) strengthened the Indonesian Throughflow (Gallagher et al., 2009; Rosleff-Soerensen et al., 2012) and are thought to have extended carbonate production up to 29°S, along Australia, by the Holocene (Zhu et al., 1993).
Regional Stratigraphy and Associated Petroleum System
The seismic-stratigraphic framework of the Browse Basin is summarized in Figure 2. The Browse Basin petroleum system consists of Lower–Middle Jurassic source rocks, namely, fluvio-deltaic to marine coals and shales deposited during the synrift stage (Fig. 2; Tovaglieri and George, 2014). These Jurassic source rocks reached maturity and started expelling hydrocarbons during the Miocene (Blevin et al., 1998a; Grosjean et al., 2016). The Lower–Middle Jurassic Plover Formation (synrift) forms the major sandstone reservoir in the Browse Basin. Postrift strata include Upper Jurassic organic-rich marine shales (Stephenson and Cadman, 1994; Rosleff-Soerensen et al., 2012; Grosjean et al., 2016), and interbedded Lower Cretaceous shales and carbonates with thin hydrocarbon-bearing turbidites (Stephenson and Cadman, 1994).
Argillaceous carbonates became prevalent in the Browse Basin during the Paleocene (Fig. 2; Apthorpe, 1988). An Eocene–early Oligocene carbonate ramp developed before a stratigraphic break recording uplift and erosion of the inner shelf (Fig. 2; Stephenson and Cadman, 1994; Howarth and Alves, 2016; Belde et al., 2017). Thereafter, a relative sea-level fall resulted in the accumulation of shallow-marine carbonates and the evolution of a late Oligocene–Miocene rimmed platform (Fig. 2; Apthorpe, 1988; Howarth and Alves, 2016; Belde et al., 2017). In summary, the Miocene and Pliocene sections of the Browse Basin represent an aggradational carbonate shelf. The Miocene rimmed platform was drowned in the late Miocene and largely buried by Pliocene hemipelagic sediment (Rosleff-Soerensen et al., 2012; Belde et al., 2017), whereas the modern-day Seringapatam Reef is locally emergent (Fig. 2; ConocoPhillips, 2012).
The reactivation of Jurassic faults and regional tectonic inversion that occurred after the onset of subduction in the Timor Trough resulted in highly variable deformation styles across the Browse Basin (Harrowfield and Keep, 2005). Small-scale normal faults crosscut late Oligocene–Miocene strata in the eastern part of the study area, forming east-trending lineaments on the sea floor (Howarth and Alves, 2016). While no other faults offset the Miocene sequence, it is noted that sub-seismic-scale fractures and faults may be present and control fluid flow. In addition, late Miocene inversion is documented in the southern Browse Basin (Rosleff-Soerensen et al., 2016).
In parallel with other studies on the North West Shelf, tectonic inversion has been used to suggest faulting as the key control on Cenozoic carbonate growth offshore Northwest Australia (Saqab and Bourget, 2015; Courgeon et al., 2016; Rosleff-Soerensen et al., 2016). However, Howarth and Alves (2016) found no evidence in the study area for direct control by underlying faults on the initiation or distribution of isolated carbonate buildups.
Under the right conditions, the dissolution of carbonate rocks by water can develop extensive porosity (Field, 1999; Wright et al., 2014). As such, karsts and paleokarsts represent key plays within carbonate-hosted reservoirs worldwide, namely, in hydrocarbon fields in China (Han et al., 1998; Liu et al., 2004), Texas (Loucks and Anderson, 1985; Kerans, 1988), Thailand (Heward et al., 2000), Middle East (Harris et al., 1984; Lindsay et al., 2006), and in the “Golden Lane,” Gulf of Mexico (Carrasco, 2003). Occurring at multiple scales, karsts are commonly observed along regional unconformities and platform margins due to dissolution of carbonate promoted by groundwater in the meteoric vadose and phreatic zones. An additional process generating karsts is the enhanced mixing of meteoric waters with sea water (mixing corrosion) on platform margins (Wright et al., 2014).
Karsts are regularly identified on seismic data by computing variance maps and time slices. They form negative-relief features with high variance, and are often circular in shape. In some cases, karsts generate a network pattern, as described in Collon-Drouaillet et al. (2012). Such a pattern is often dendritic (or branchwork) on attribute maps.
Howarth and Alves (2016) identified three karst horizons within the Upper Oligocene–Miocene sequence of the Browse Basin. The greatest density of karsts occurring to the southeast, above the inner ramp of the platform, was interpreted to mark prolonged subaerial exposure. Smaller populations of karsts were recorded on the platform margin and northwest edges of isolated carbonate buildups, and these likely represent short-lived exposure events (Howarth and Alves, 2016).
Fluids move buoyantly within a sediment body, both laterally and vertically. The migration pathways for fluids are unique to each basin and are a function of fracture flow, Darcy flow, and diffusion (Kroos and Leythaeuser, 1996). The point where fluid reaches the sea floor, or the surface, is termed a “cold seep” (Judd and Hovland, 2007).
Fluid-flow features are commonly observed on tectonically active continental shelves, settings that are characterized by high sedimentation rates, or they occur near petroleum fields where sediment is overpressured (Osborne and Swarbrick, 1997; Rogers et al., 2006). Key examples include the Gulf of Cádiz (Somoza et al., 2003), offshore Malaysia (Fathiyah Jamaludin et al., 2015), central North West Shelf of Australia (Jones et al., 2006; Rollet et al., 2009; Logan et al., 2010), and offshore Namibia (Moss and Cartwright, 2010). Fluid migration follows the path of greatest permeability, either exploiting differences in porosity and permeability between carbonate facies, or due to the effect(s) of faults and fractures on seal capacity, with or without juxtaposition of permeable strata on either side of a regional seal (Løseth et al., 2009; Frazer et al., 2014; Serié et al., 2016). Similarly, pore-fluid pressure gradients can develop between lithologies with different compressibility characteristics (e.g., carbonates and compressible fine-grained basin sediments), where fluids are laterally forced out of compressible units (Frazer et al., 2014).
The identification and evaluation of fluid-flow pathways are key steps in reducing hydrocarbon exploration uncertainty (Cartwright, 2007; Moss and Cartwright, 2010). In the Browse Basin, O’Brien et al. (2005) documented seeping dry gas and oil on the sea floor of the Yampi Shelf, which provides evidence for hydrocarbon flow to the southeast of the study area.
Fluid Flow and Isolated Carbonate Buildups
Methanogenic carbonates either form seepage-associated carbonates or chemoherm carbonates. The latter form a buildup of precipitated chemical carbonates with the calcareous skeletal debris of chemosynthetic fauna producing a structure up to 90 m high (Teichert et al., 2005) under sustained seepage of methane-rich fluids into bottom waters (Han et al., 2008). Seepage-associated carbonates occur at the sea floor or subsurface as carbonate slabs, concretions, crusts, and tubes, interpreted to represent slower fluid-flow regimes (Han et al., 2008).
Carbonates formed by anaerobic oxidation of methane usually occur in deep, cold, and anoxic waters (Michaelis et al., 2002; Aloisi et al., 2002; Wild et al., 2015). Few have been documented in shallow tropical waters (e.g., Wild et al., 2015), because oxic conditions cause aerobic oxidation of CH4 and the widespread production of CO2. However, this does not mean methanogenic carbonates cannot occur in oxic conditions, as methane anoxia can create localized anoxic environments near fluid pathways (Wild et al., 2015).
DATA AND METHODS
This work uses a 3-D seismic volume covering 2851 km2 of the Browse Basin, on the shelf margin (Fig. 1). The seismic survey was acquired in a direction parallel to the northwest-striking continental shelf and is not aligned with the long axis of fluid-venting structures, or with any karst networks. Therefore, no spatial aliasing occurred when imaging shallow features (Ho et al., 2012). The data set follows the Society of Exploration Geophysicists (SEG) European polarity convention, i.e., an increase in acoustic impedance equals a red reflection of negative amplitude (Rosleff-Soerensen et al., 2016).
The interpreted data set includes two exploration wells, Poseidon-1 and Poseidon-2 (Fig. 1). Poseidon-1 is a wildcat and recorded the first gas discovery in the Browse Basin (Poseidon field). It provided gamma-ray data, including the rate of penetration and resistivity data from 4000 to 6000 meters below the sea floor (mbsf). The Poseidon-2 well was drilled to assess the presence and nature of the gas in the Plover Formation. It provided lithological data below 2430 mbsf (Howarth and Alves, 2016). Both wells cross the Upper Oligocene–Miocene sequence, but neither sampled an isolated carbonate buildup directly. As a result, isotope data used in this work was acquired at the Brecknock-1 well (Fig. 1), which sampled the same carbonate sequence ∼75 km to the southwest of the study area (Rosleff-Soerensen et al., 2016).
Faults were mapped manually every 10 inlines and crosslines. Ninety-one (91) pinnacle features were also mapped in areas where seismic reflections exhibited a sharp, conical geometry (Fig. 3). They are distinct from velocity pull-ups, which are the result of sharp subsurface velocity contrasts because they comprise localized features within larger isolated carbonate buildups and overlie flat seismic reflections (Fig. 3). Horizon mapping for every inline and crossline was undertaken to image the morphology of pinnacle features (Fig. 3).
The presence of fluid in subsurface strata was interpreted through the identification of seismic anomalies. These included bright spots, flat spots, phase reversal associated with accumulated hydrocarbons, and hydrocarbon-related diagenetic zones (O’Brien and Woods, 1995; Logan et al., 2010). Vertical dim zones and vertical bright zones were also mapped, because they result from the scattering, attenuation, and decrease in the velocity of compressional waves (P waves) as they pass through gas-charged zones (Løseth et al., 2009; Fathiyah Jamaludin et al., 2015).
Mud volcanoes and pockmarks are surface expressions of fluid flow (Løseth et al., 2009; Moss and Cartwright, 2010), and were interpreted in the study area. Mud volcanoes exhibit circular to subcircular conical geometries, with slope angles up to 30° and a flat top that generates a high-amplitude reflection (Somoza et al., 2003). Pockmarks are crater-shaped erosional depressions formed in soft fine-grained seafloor sediment (Hovland and Judd, 1988).
Velocity pull-ups and push-downs are caused by contrasts in the seismic velocity of distinct lithologies, e.g., carbonate and shales. To accurately interpret velocity pull-ups as fluid-flow features, the areas showing seismic anomalies were compared to known examples of fluid flow (Jones et al., 2006; Rollet et al., 2009; Logan et al., 2010; Fathiyah Jamaludin et al., 2015).
The variance attribute was applied to the seismic volume to highlight discontinuities in, or between, seismic reflections that generated high variance (Howarth and Alves, 2016). As such, it was useful for imaging subtle stratigraphic features that generated 3-D offsets, such as karsts, faults, and fluid pipes (Omosanya and Alves, 2013; Marfurt and Alves, 2015).
Structural mapping and variance data revealed three fault families (Fig. 4, supplementary Figs. DR1–DR3).1 The first family consists of northeast-striking extensional faults that are parallel to the shelf margin and that dip northwest and southeast (Fig. 4 and DR1 [see footnote 1]). These faults are laterally continuous features that cause large displacements within pre-Cenozoic strata and generate significant fault block topography (Fig. 2). Faults tip-out within overlying Cretaceous strata, but they were not observed to offset Paleocene units.
The second family of faults also consists of northeast-striking extensional structures parallel to the shelf margin. They are confined to the break in slope of the Eocene–early Oligocene ramp (Figs. 2, 4, and DR2 [see footnote 1]). None of the faults observed in this family was related to the position of isolated carbonate buildups or appeared to influence isolated carbonate buildup morphology (Fig. 4).
The third fault family consists of east-striking extensional faults that are near-perpendicular to the shelf margin. These faults offset Pliocene to Eocene strata and show relatively small throws. No growth structures were observed within Upper Oligocene–Miocene strata, and faults in this family were confined to the eastern part of the study area (Figs. 2, 4, and DR3 [see footnote 1]).
Within the Lower Oligocene–Miocene sequence, four (4) horizons are characterized by chaotic to discontinuous seismic reflections (Fig. 5). Lateral interruptions in seismic reflections generate high-variance, negative-relief features that are greater than 60 m in diameter. Horizon 1 occurs from –1750 ms to –1900 ms across the top Eocene boundary into Lower Oligocene–Miocene strata. It is characterized by pervasive circular topographic depressions (Figs. 5A and 5B). In a number of examples, this negative relief is linear and shows a branchwork pattern across the ramp interior. Seismic reflections become continuous along this horizon. To the southwest, they develop sigmoid to sigmoid-oblique geometries, which reveal successive phases of progradation toward the southeast (Fig. 5).
Horizon 2 also shows discontinuous seismic reflections associated with high-variance negative topography clustering within the isolated carbonate buildups (Fig. 5). Discrete, high-variance, circular depressions commonly reveal a branchwork pattern, with individual depressions showing a minimum diameter of 60 m (Figs. 5A and 5B).
Horizon 3 documents the same character to horizons 1 and 2 (Fig. 5). Growth patterns show build-out and build-down geometries, while isolated carbonate buildups are characterized by discontinuous seismic reflections in the northern part of the study area.
Within the Upper Oligocene–Miocene sequence, 91 discrete seismic reflections exhibit circular to subcircular, conical geometries (Fig. 3). We refer to these features as pinnacles or pinnacle features.
Pinnacles are high-amplitude, continuous seismic reflections that are onlapped by low-amplitude strata (Figs. 3 and 6). Pinnacles typically have slope angles reaching a maximum of 8°. Their height ranges from 31 m to 174 m, with an average of 77 m and a mode of 66 m (Fig. 7A). The histogram in Figure 7B shows a normal distribution for pinnacle heights with a positive skew. Interpreted pinnacles range in volume from 33 km3 to 11,105 km3, with an average of 2424 km3 and a mode of 780 km3 (Fig. 7A). The histogram in Figure 7C reveals a strong positive skew, with a sharp decline in the frequency of pinnacle exceeding 3000 km3 in volume.
Internally, pinnacle features show relatively low amplitudes, but some reveal evidence of layering. However, the interpretation of their internal character becomes increasingly difficult as pinnacle reflection size decreases, especially where pinnacle diameters are at the lower limit of seismic resolution. When separated according to depth, pinnacle features show a linear northeast trend during the early stages of isolated carbonate buildup growth (Fig. 8). Importantly, this linear trend follows the break in slope of the underlying carbonate ramp.
Subsequent pinnacle features appear basinward to the northwest and show increasingly clustered distributions within isolated carbonate buildup interiors on the basin margin, and in the northeast of the study area (Figs. 8, 9, 10, and 11). Their positions mirror isolated carbonate buildups and the locations of karst horizons (Figs. 5, 8, 9, and 10). Pinnacle features are scarce within parallel, low-amplitude reflections that onlap isolated carbonate buildups, notably within a northwest-trending corridor in the center of the study area, as well as in the southeast (Figs. 8, 9, 10, and 11).
Pinnacles are commonly onlapped by seismic reflections over which larger isolated carbonate buildups have developed (Fig. 6). Seismic reflections in the larger isolated carbonate buildups exhibit sigmoidal to sigmoidal-oblique geometries (Fig. 6). However, significant variations in their geometry were observed when individual isolated carbonate buildups were compared.
Fluid Flow and Associated Seismic Anomalies
Vertical to subvertical zones of discontinuous and suppressed seismic reflections (vertical dim zones) were observed throughout the 3-D seismic volume (Figs. 8, 9, 10, and 11). The spatial distribution of vertical dim zones beneath the top ramp horizon appears random in the southeast of the study area, but shows an increasingly linear distribution to the northwest (Fig. 8). This linear distribution reveals a northeast trend to vertical dim zones that mirrors the strike of pre-Miocene fault families (Fig. 8). Furthermore, this northeast trend matches the strike of progradational clinoforms within the Eocene–early Oligocene ramp (Fig. 8).
The spatial distribution of vertical dim zones becomes increasingly clustered moving upwards through the Upper Oligocene–Miocene sequence, focusing within isolated carbonate buildup interiors along the basin margin (Figs. 8, 9, 10, and 11). Throughout the Upper Oligocene–Miocene sequence, the distribution of vertical dim zones is associated with isolated carbonate buildups (Fig. 8). This is particularly shown in Figure 9, where low-variance and low-amplitude strata bury part of the large shelf margin. In contrast, vertical dim zones and pinnacles are no longer observed within the areas buried by low-variance seismic facies in the platform interior, or along a northwest-trending transect in the middle of the study area (Figs. 8, 9, 10, and 11). Instead, vertical dim zones and pinnacles occur within the high-variance isolated carbonate buildups (Figs. 8, 9, 10, and 11).
The majority of vertical dim zones terminate within Pliocene strata (Fig. 11). Smaller numbers of localized amplitude anomalies, pockmarks, and mud volcanoes are observed in Pliocene–Quaternary strata and on the sea floor, particularly around the Seringapatam Reef (Fig. 12). In addition to vertical dim zones, seismic washouts are observed within steep clinoforms on the northwest margins of isolated carbonate buildups and their interior (Fig. 12). Their presence is highlighted by a notable increase in the amplitude of seismic reflections on the southeast margins of isolated carbonate buildups, which typically show a more continuous nature (Fig. 12). In these areas, bright spots (Fig. 12) are often laterally continuous and range typically from 100 m to ∼1 km in length. In some cases, bright spots are vertically stacked around vertical dim zones (Fig. 12). Bright spots can also be seen in horizons characterized by discontinuous seismic reflections, as well as vertical dim zones (Fig. 12).
Figure 12 shows conical topographic highs on the modern sea floor that are characteristic of mud volcanoes and steep-sided depressions suggestive of pockmarks. These features are observed above vertical dim zones, bright spots, and areas of seismic washout, particularly around the Seringapatam Reef. In parallel, velocity pull-ups are observed beneath the margins of the largest isolated carbonate buildups, which are onlapped by lower-amplitude horizontal reflections (Fig. 6). Underlying seismic reflections dip upward and mirror the geometry of the overlying isolated carbonate buildup. In some cases, a discontinuous character comparable to vertical dim zones is observed beneath the margins of isolated carbonate buildups (Fig. 6C).
Relationship between Pinnacle Features and Seismic Anomalies
A causal relationship is thus recorded between pinnacle features and vertical dim zones (Figs. 3 and 8). During the initial stages of isolated carbonate buildup growth, pinnacle features are observed directly above the areas with the highest concentration of seismic anomalies (Figs. 8 and 9). Thereafter, the spatial distribution of vertical dim zones and other seismic anomalies correlates with the position of the Miocene isolated carbonate buildups (Fig. 10).
Pinnacles are not commonly observed within the thick units of low-amplitude, laterally continuous onlapping reflections against which vertical dim zones often terminate (Figs. 3 and 9). In addition, not all pinnacles sit above seismic anomalies. Some sit beneath the margins of isolated carbonate buildups (Fig. 9), while others sit above strata lacking seismic anomalies (Fig. 6B).
Where Are the Main Sources for Fluids in the Browse Basin Located?
Our results document a number of seismic anomalies that resemble fluid-flow features recorded elsewhere on the North West Shelf (Fig. 12; O’Brien and Woods, 1995; Jones et al., 2006; Rollet et al., 2009, Logan et al., 2010). The distribution of fluid-flow features in specific areas of the late Oligocene–Miocene platform, and their association with pre-Miocene faults (Fig. 12) could be attributed to a biogenic origin. However, when analyzed in combination with the Browse Basin petroleum system, the distribution of fluid-flow features in the study area is suggestive of deeper fluid plumbing (Serié et al., 2016). Hydrocarbon leakage from deep reservoirs is a common occurrence in the Timor Sea (Gartrell et al., 2003), Malay Basin (Ghosh et al., 2010), Australian Basin (Logan et al., 2010), and other hydrocarbon-producing areas worldwide (Schroot et al., 2005; Fathiyah Jamaludin et al., 2015). In the Browse Basin, hydrocarbon migration is believed to have started as early as the latest Cretaceous (Blevin et al., 1998b) and continued through the Neogene (Grosjean et al., 2016). In parallel, modeling results show that the westerly flexure of the shelf favored hydrocarbon migration toward the basin margins where the late Oligocene–Miocene platform was developing (Blevin et al., 1998a). As Cretaceous shales, comprising the primary seal for a number of plays in the Browse Basin, exhibit variable thickness across the basin (Blevin et al., 1998a), doubts can be raised about its effectiveness as a regional seal interval. We suggest that fluid was likely breaching this seal and reaching successive paleo-seafloors up to, and throughout, the late Oligocene and Miocene.
Blevin et al. (1998a) recognized a number of structural and stratigraphic traps in the Browse Basin. Our results show a correlation between the position of vertical dim zones and vertical bright zones and the deeper extensional faults (fault family 1), suggesting that these faults acted as migration pathways for fluid (Figs. 3 and 8). However, fault family 1 does not offset the Eocene–early Oligocene ramp (Fig. 2). As such, their control on fluid flow is expected to diminish away from their upper tips. Faults in extensional fault family 2, within the prograding clinoforms of the Eocene–early Oligocene ramp (Figs. 4 and 13A), do not extend into the Upper Oligocene–Miocene sequence and relate to syndepositional deformation (Rankey, 2017). Fluid-flow distribution suggests this fault family likely created a preferred migration pathway through the Eocene–early Oligocene ramp (Fig. 8).
Comparisons between the distribution of fluid-flow features and depositional facies models for carbonate ramps, as described by Wilson (1975), indicate that fluid migration follows higher-permeability grain-rich facies belts (e.g., boundstones, bafflestones, and grainstones) on the wave-agitated inner ramp (Fig. 13; Ahr, 1973; Read, 1985). It must be noted, however, the striking difficulty in ascertaining the permeability of ramp facies from seismic data alone. The termination of fluid flow features against onlapping, low-amplitude seismic reflections northwest and southeast of the initial isolated carbonate buildup growth sites is interpreted to highlight the seal potential of muddier, finer-grained, lower-permeability facies in outer-ramp and lagoonal areas (Figs. 8, 9, 10 and 11; Ahr, 1973; Read, 1985). When considered in combination with the northwest dip of the Eocene–early Oligocene ramp (Figs. 5C and 13A), this potentially sealing facies facilitates updip migration along bedding planes toward the break in slope, upon which isolated carbonate buildup growth was initiated (Figs. 5C, 8, and 13A).
Based on the observed correlation between fluid-flow features and karstified intervals (Figs. 8, 9, 10, and 11), we suggest that karstification influenced fluid flow by generating secondary permeability and porosity similarly to oil and gas fields in China (Han et al., 1998; Liu et al., 2004), Texas (Loucks and Anderson, 1985; Kerans, 1988), Thailand (Heward et al., 2000), and other carbonate platforms worldwide (Loucks, 1999; Fournillon et al., 2012). We cannot determine permeability and porosity from seismic data alone, as karstification and diagenetic processes associated with the emergence of isolated carbonate buildups are highly variable.
What Do Pinnacle Features Observed at the Base of Isolated Carbonate Buildups Reflect in Terms of Geological Processes?
In the Browse Basin, the spatial relationship between fluid-flow and pinnacle features indicates that pinnacles are a surface expression of paleo–fluid flow (Figs. 6 and 8; Hovland and Judd, 1988; Milkov, 2000). The conical geometries of pinnacle features match the diagnostic criteria for mud volcanoes as described in Brown (1990) and Kopf (2002); see Figure 3. When compared to the classification scheme of Kalinko (1964), which identified three classes of mud volcano based on the character of their activity with respect to morphological expression (Dimitrov, 2002), the interpreted pinnacle features are suggestive of class 1, Lokbatan-type mud volcanoes. Formed by periodic explosive activity, due to the buildup of pore fluid pressure beneath a blockage in the feeder channel, they extrude low-viscosity mud breccia to form a steep conical-shaped mud volcano (Fig. 14B; Kalinko, 1964; Dimitrov, 2002).
The sizes and slope angles of pinnacle features in the Browse Basin are consistent with mud volcanoes mapped in southwest Taiwan (Chen et al., 2014) and in the Gulf of Cádiz, where single, circular mud volcanoes have bathymetric relief of 80–100 m and slope angles of 6° to 8° (Somoza et al., 2003). Their internal seismic character (Fig. 3) reveals a strong top reflection surface, also characteristic of mud volcanoes in the Gulf of Cádiz (Somoza et al., 2003), where a sharp lithological change occurs between the mud volcano and overlying carbonate sediments (see also Barber et al., 1986; Pickering et al., 1988; Orange, 1990). However, mud volcanoes are not commonly documented in Miocene strata of the North West Shelf (Jones et al., 2006; Logan et al., 2010), probably due to strong ocean currents and wave activity not allowing the formation of stable mounds of the heights we document (Logan et al., 2010; Bachtel et al., 2011). Nevertheless, we documented mud volcanoes on the modern sea floor with heights and slope angles that are comparable to pinnacle features in the Upper Oligocene–Miocene sequence (Fig. 12A). Considering that the present-day North West Shelf is a high-energy, shallow-water environment that experiences macrotides, seasonal cyclonic storms, and long-period swells (James et al., 2004), the presence of pinnacles on the modern sea floor suggests that structures associated with seepage (e.g., pockmarks, mud volcanoes) were not quickly destroyed by currents and waves in the late Oligocene–Miocene Browse Basin (Logan et al., 2010). Instead, the pinnacle features interpreted at depth survived long enough to be colonized by reef-building organisms.
The potential presence of hydrocarbon-charged fluids in the Browse Basin, when considered together with regional compression, provides a driving force for mud volcanism by decreasing mud density and increasing buoyancy forces (Hovland and Curzi, 1989; Brown, 1990; Rollet et al., 2009). Yet, questions remain as to the source of mud for the >20-m-high mud volcanoes in the Browse Basin. Examples of mud volcanoes from southwest Taiwan sit above mud diapirs (Brown, 1990; Milkov, 2000; Talukder et al., 2007). In the Browse Basin, mud volcanoes sit above a thick unit of hemipelagic sediment on the modern sea floor (Fig. 12A). Conversely, no mud diapirs are observed in the study area within pre-Miocene strata. Pinnacles observed above the Eocene–early Oligocene ramp, and within late Oligocene–Miocene isolated carbonate buildups, notably occur within porous and permeable facies (Wilson, 1975) and contain only a small percentage of mud (Figs. 3, 6, and 8).
Potential sources of mud could come from horizontal seismic reflections within isolated carbonate buildups, e.g., lagoonal muds (Bachtel et al., 2004). Alternatively, during transgressive periods, isolated carbonate buildups may have been onlapped by parallel, flat reflections, exhibiting low variance (Figs. 8, 9, and 10), suggesting they were draped by fine-grained basinal muds (Bachtel et al., 2004). These muds may also have provided a source of mud, or acted as a barrier to flow, against which pore fluid pressure was able to increase (Kalinko, 1964). This seems to contradict studies indicating that mud volcanoes do not form above carbonate strata (Margreth et al., 2011).
An alternative interpretation is that pinnacle features represent methane-derived carbonates (Hovland, 1990), as suggested for carbonate deposits at contemporary and ancient methane seeps (Fig. 14; Mullins and Neumann, 1979; Hovland et al., 1987, 1994; Hovland and Judd, 1988; O’Brien et al., 2003; Sumida et al., 2004; Reitner et al., 2005a, 2005b). Localized high-amplitude seismic reflections beneath isolated carbonate buildups and mounds (Figs. 6 and 12), often rooted on an erosional surface, are similar in character to carbonate-cemented hardgrounds in the Arafura Basin, Northwest Australia (Rollet et al., 2009). Also, pinnacle features in the study area are comparable in size and geometry to mounds up to 30 m high on the Yampi Shelf, interpreted as carbonate crusts associated with fluid flow (Rollet et al., 2009).
Carbon isotope data from the Brecknock-1 well (southern Browse Basin) record δ13C values from 0.9‰ to 2.4‰ for the Miocene carbonate sequence (Rosleff-Soerensen et al., 2012), significantly lower than typical δ13C values (up to +30‰) for methanogenic carbonates (Claypool and Kaplan, 1974; Naehr et al., 2000, 2007), but suggestive of precipitation from sea water (Naehr et al., 2000, 2007; Díaz-del-Río et al., 2003). Notwithstanding the latter interpretation, it is important to acknowledge that the Brecknock-1 well is located over 75 km to the southwest of the seismic volume (Fig. 1) and does not sample a pinnacle. The limited data available from the Brecknock-1 well are not conclusive.
A third explanation is that pinnacles represent the first seismically resolved patch reefs preceding the growth of the larger isolated carbonate buildups that characterize the late Oligocene–Miocene and, therefore, have no association with fluid flow. This would fit with the re-evaluation by Logan et al. (2010) of the data interpreted in O’Brien et al. (2003) for other parts of Northwest Australia. While pockmarks and mud volcanoes are observed on the present-day sea floor, the absence of pockmarks within the Upper Oligocene–Miocene sequence indicates fluid flow was not enough to drive mud volcano or methanogenic carbonate formation in the study area (Fig. 12). Alternatively, it can indicate that fluid flow postdated the Miocene. As such, the observed relationship between seismic anomalies and pinnacles may relate to velocity pull-ups caused by overlying isolated carbonate buildups, or by shallow faults or fractures with negligible offset (O’Brien et al., 2002; Logan et al., 2010), likely associated with regional Neogene compression (Jones et al., 2008).
Was There an Active Seep-Reef Relationship in the Browse Basin during the Miocene?
This study identifies no relationship between fault-generated topography and isolated carbonate buildup growth (Figs. 4 and 6), agreeing with Howarth and Alves (2016). Instead, we document a causal relationship between seismic anomalies and pinnacle features, which served as focal points from which larger isolated carbonate buildups developed (Figs. 6 and 14). While Logan et al. (2010) dismissed a seep-reef relationship in other parts of Northwest Australia, upon integrating the results in this paper with regional tectonics (Macgregor, 1993) and data from the local petroleum system (O’Brien and Woods, 1995; Blevin et al., 1998a; Grosjean et al., 2016), we provide evidence to support the essentially Miocene migration of hydrocarbon-bearing fluids in the Browse Basin. This migration could have generated active seepage sites on the late Oligocene–Miocene paleo-seafloor to support the formation of mud volcanoes or methanogenic carbonates.
The absence of pockmarks within the Upper Oligocene–Miocene sequence, and other studies of seep-reef relationships in the Browse Basin (e.g., Logan et al., 2010), limits the interpretation of pinnacles as fluid-flow–related features. In fact, to determine whether seismic anomalies beneath pinnacle features are false or real structures, accurate acoustic logs are required from the intervals in which they occur to generate accurate velocity models (Marfurt and Alves, 2015), data that are not provided by the Poseidon-1 and Poseidon-2 wells. However, the data in this work show that the topography generated by the pinnacle features exceeds the minimum topography of 10 m required to trigger preferential isolated carbonate buildup growth (Figs. 5, 6, and 7; Rosleff-Soerensen et al., 2016). The observation that these features appear routinely at the base of isolated carbonate buildups (Figs. 3 and 6) suggests they were in place prior to the main phase of isolated carbonate buildup growth. The fact that not all pinnacle features developed larger isolated carbonate buildups above, particularly in the deeper parts of the basin and carbonate platform, is a proof that the topography generated by the pinnacles was not always sufficient to raise the sea floor into the photic zone, where reef-building organisms could eventually establish. It thus shows that pinnacles were capable of placing the sea floor in the photic zone only where the Browse Basin shelf was at its shallowest (Fig. 12).
Hence, the model favored in this paper is one in which hydrocarbon-bearing fluids from deeper reservoir intervals provided enough methane and sulfate to stimulate precipitation through anaerobic oxidation of methane below the sediment-water interface (Reitner et al., 2005a, 2005b; Han et al., 2008). As a result, precipitation of a bottom plate of hard sediment could have ensued, upon which “tower” or “mounded” methanogenic carbonates (Reitner et al., 2005a, 2005b; Rollet et al., 2009) generated the pinnacle features observed in the Upper Oligocene–Miocene sequence (Fig. 14). A limitation to this model is that methanogenic carbonates are commonly restricted to anoxic conditions (Peckmann et al., 2001), and planktonic assemblages from the Browse Basin indicate that the early Miocene platform experienced shallow, warm, and oxic conditions (Rosleff-Soerensen et al., 2012; Howarth and Alves, 2016). These conditions cause aerobic oxidation of methane and dissolution of methanogenic carbonates, rather than their precipitation (Teichert et al., 2005). Thus, for methanogenic carbonates to have formed in the Browse Basin, either: (1) stratification of the water column occurred during the late Oligocene–Miocene, allowing for the formation of anoxic bottom waters, or (2) communities of archaea and sulfate-reducing, bacteria-coated chemoherms protected them from oxic seawater conditions, and allowed methanogenic carbonate production within the underlying anoxic microenvironment. This same process has been proposed for methanogenic carbonates at Hydrate Ridge, offshore western North America (Teichert et al., 2005).
The main conclusions of this study are:
(1) Pre-Paleogene fault systems focused fluid flow from Mesozoic reservoirs into Paleogene strata. Above these reservoirs, depositional facies within prograding clinoforms of a Eocene–early Oligocene carbonate ramp and late Oligocene–Miocene isolated carbonate buildups controlled fluid flow in the study area.
(2) Seismic anomalies within Upper Oligocene–Miocene strata lead us to infer that fluid flow, and surface seepage of hydrocarbon-bearing fluids, was common on the paleo-seafloor of the Browse Basin.
(3) Pinnacle features can be interpreted as mud volcanoes, methanogenic carbonates, or the first patch reefs preceding the larger isolated carbonate buildups. Given the tectonic setting and evolution of the petroleum system in the study area, their association with fluid flow is not ruled out.
(4) Based on the results in this work, and on the current understanding of the Browse Basin’s carbonate system, pinnacles represent the first isolated patch reefs formed prior to the larger isolated carbonate buildups, with underlying seismic anomalies deriving from velocity pull-ups or small-scale fractures through which fluid was able to migrate.
(5) In the absence of fault-generated topography, pinnacles generated sufficient antecedent topography to trigger the preferential settlement of reef-building organisms, and thus controlled the distribution of isolated carbonate buildups in the Browse Basin.
The work contained in this paper is part of a Ph.D. study supported by the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil and Gas and cosponsored by Cardiff University. Geosciences Australia is acknowledged for the provision of data for this research paper. We acknowledge Schlumberger (Petrel©) for granting academic licenses to Cardiff’s 3-D Seismic Laboratory. We thank the two anonymous reviewers and Cathy Hollis for their suggestions and improvements to initial versions of this work.