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Hydrocarbon potential of the mesozoic Carbonates of the Bahamas
Abstract and Figures
In the early 1980’s, the Getty Oil Company developed petroleum play concepts for several lease areas that it controlled in the Bahamas. Interest in these areas was based upon the facts that both Cuba and Florida had producing oil fields, oil shows were reported in test wells that had been drilled in the Bahamas and the well logs from these test wells indicated that the amount of evaporites in the Bahamas Platform carbonate-evaporite complex increased with depth. Evaporites indicate arid and often anoxic and reducing depositional environments which is favorable to the preservation of organic material.
The Getty frontier exploration team addressed the formation and occurrence of source rocks, the thermal maturation histories of the potential source rocks, the presence of reservoir quality rock and possible migration pathways. The southern Bahamas was identified as the area that probably had the greatest amount of mature source rock. Thermal maturation (Lopatin) profiles for the Cay Sal well indicated that the Jurassic and Lower Cretaceous were within the oil generation window and the Upper Cretaceous was immature. Thermal maturation (Lopatin) profiles for the Long Island well indicated that the Lower Jurassic was in the gas generation window, the Upper Jurassic and Lower Cretaceous were within the oil generation window and the Upper Cretaceous was immature. Thermal maturation (Lopatin) profiles for the Great Isaac well indicated that the Jurassic and Lower Cretaceous were within the early oil generation window and the Upper Cretaceous was immature. Dolostone, with estimated porosities of between 10% and 18% were thought to be likely reservoirs. Secondary reservoir objectives were skeletal, oolitic and reefal limestones. Evaporites, mainly in the form of anhydrites, and nonporous carbonates were identified as probable seals.
Successful deep wells in the Gulf of Mexico are targeting subsalt structures at depths greater than 20,000 feet (6,100 m) (McMoran Exploration Co., 2009). The presence of significant hydrocarbons and reservoir quality rock at these depths, coupled with advances in seismic imaging, has opened up the possibility of the existence of other successful deep plays in frontier areas, such as the Bahamas.
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... Early Jurassic rifting caused marine water influx, creating shallow water conditions (Epstein & Clark, 2009;Iturralde-Vinent et al., 2016;Ladd & Sheridan, 1987;Moretti et al., 2003;Walles, 1993). Middle Jurassic syn-rift redbeds were succeeded by evaporites in hypersaline lagoons (Figure 2b; Gaumet et al., 2004;Padilla y Sánchez, 2016). ...
The Eastern Cuban block has experienced a complex tectonic history characterized by plate interactions, resulting in a diverse array of geological features observable in the offshore sedimentary record. We investigate the tectonic evolution of offshore Eastern Cuba, specifically in the Old Bahamas Channel and its surrounding areas, by integrating multi‐channel seismic (MCS) reflection and published geological data. Our analysis employs stratigraphic frameworks and MCS data to assess deformation and key geological events in the region. We highlight the complex tectonic history of the Eastern Cuban block, marked by significant geodynamic events, such as rifting, the subduction of the oceanic Proto‐Caribbean plate, and syn‐orogenic and post‐orogenic phases. The seismic units observed in the majority of the study area reveal the early evolution of the Northern Proto‐Caribbean margin, subsequently impacted by the Cuban orogeny and the reactivation of the Cuban Transform Fault zone corresponding to a former plate boundary. We propose estimated ages for the seismic sequences, correlating them with available well data from neighboring regions. This study offers valuable insights into the tectonic history and geological evolution of offshore Eastern Cuba, contributing to a more comprehensive understanding of the region's geodynamic development.
... However, this understanding has always been challenged by the existence of deep petroleum reservoirs. The Smackover dolostones deeper than 5100 m still have an average porosity of 15% in the Jay Field (Melas and Friedman, 1992); the Great Isaac 1 well encountered live oil in the dolostone reservoir of 10% to 18% porosity and 5390 m depth in the southern Bahamas area (Epstein and Clark, 2009); dolostones in the Delaware basin have up to 12% effective porosity at a depth of 6477 m (Amthor and Friedman, 1991). ...
Discoveries of deep high‐quality carbonate reservoirs challenged the general knowledge on the evolution of porosity which decrease with deepening. New mechanisms of pore generation and preservation in the deep realm need to be proposed. Dolostones in the Feixianguan and Dengying Formations experienced maximum depth excess 8000 m, but still retain high porosities. Petrographic observation and homogenization temperatures help to identify products of deep fluid‐rock interactions, visual and experimental porosity were used to quantify reservoir effects, finally the distribution of products was plotted to unravel the mechanisms. Th data reveal that thermochemical sulfate reduction (TSR), burial dissolution and quartz cementation are the typical deep fluid‐rock interactions. The SO42− of residual porewater sourced from the evaporative dolomitizing fluid was supplied for TSR in the hydrocarbon column, the TSR‐inducing calcite cements were homogeneously dispersed in the hydrocarbon column. Quartz cementation was caused by the increasing acidity and Si‐rich residual porewater in the oil column. Burial dissolution is forced by organic acid and limited in the oil–water contact. This study suggests that seal and source rocks not only play important roles on hydrocarbon accumulation, but also have general control on the deep fluid‐rock interactions and porosity evolution in the deep burial realm.
... Potentially, the solid-state reordering of 13 C-18 O, which could influence the amount of 13 C-18 O in the carbonate mineral crystal lattices, could occur if the ambient temperature around the carbonate rocks was >100°C over a period of tens of millions of years (Dennis and Schrag, 2010;Passey and Henkes, 2012;Henkes et al., 2014). Geothermal gradients reported from isolated Cenozoic islands range from~−20°C·km −1 for Enewetak Atoll in the equatorial Pacific Ocean (Saller, 1984),~10°C·km −1 for the Nansha Islands in the South China Sea , to~15°C·km −1 for the Bahamian platform (Epstein and Clark, 2009). Based on these figures, especially the geothermal gradients of Nansha Islands in the South China Sea , the deepest part of the Huangliu Formation in CK-2 (~520 m) probably experienced a maximum burial temperature of no more than 50°C. ...
Carbonate clumped isotope thermometry is a relatively new type of analysis that has been used to obtain a better understanding of the temperature and salinity of the water that mediated development of dolostones in the Cenozoic successions of isolated oceanic islands. There is, however, still some uncertainty about the factors that could modify the clumped isotopes even in shallow burial conditions and thereby affect the temperatures and salinities derived from them. Here, focus is placed on a 210.5 m thick succession of Miocene dolostones that is found on the Xisha islands in the South China Sea that have experienced rapid subsidence since the Early Miocene. These dolostones are typically formed of crystals that have a “dirty” core that is encased by a clear outer rim. The formation of the “dirty cores”, the primary replacement dolomite, was probably mediated by normal seawater in near-surface conditions, whereas the later cements formed at higher temperatures from slightly evaporated seawater. Given that it is impossible to separate the “dirty cores” from the clear rimes for analytical purposes, it must be recognized that the clumped isotopes and other geochemical data (e.g., δ¹⁸O, ⁸⁷Sr/⁸⁶Sr) are averages of these two types of dolomite. This also means that the development of dolomite in these dolostones involved two successive stages of dolomite development that was probably related to sea-level cycles. Despite these issues, clumped isotope thermometry can still provide valuable temperature and salinity information for understanding the dolomitization processes that led to the development of island dolostones.
... Based on experimental data, approximately 100° C has been suggested as the minimum temperature under which solid-state reordering can alter primary Δ 47 values over the ~100 million year timescale evaluated here Henkes et al., 2014;. Under no realistic geothermal scenario have samples above 1500 m depth experienced temperatures of 100°C, and measurements suggest this threshold is reached closer to 4 km depth (Epstein and Clark, 2009). ...
This dissertation focuses on how mineralogical differences affect original clumped isotope (Δ47) values and how they are changed through diagenesis. A primary motivation for this work was to understand the geologic conditions under which the clumped isotope thermometer can be applied with confidence to paleoclimatic problems. To place empirical constraints on the conditions under which Δ47 alteration can occur, samples from a ~4500 m long drill core from Andros Island, Bahamas were analyzed. These limestones and dolomites formed under near-surface temperature conditions, but for samples below ~1.3 km in depth, calculated Δ47 temperatures increase by ~10° C. This indicates a shift from preserved near-surface temperatures to diagenetically modified values. Importantly, this shift is not accompanied by changes in common diagenetic indicators, and thus raises the possibility of solid-state clumped isotope alteration occurring at much lower temperatures than previously thought. Similar Δ47 temperatures recorded by each carbonate phase suggest that fine-grained dolomites and calcites are equally viable materials for recording surface temperature conditions, but both are also equally susceptible to alteration of their primary clumped isotope abundances when buried. A challenge remaining in clumped isotope thermometry was the lack of an empirical calibration for dolomite, which had been theoretically predicted to differ from established calibrations. Here, analysis of a set of synthetic and natural dolomites formed at known temperatures results in a calibration line statistically indistinguishable from calcite clumped isotope calibrations. This supports the idea of a universal calibration for carbonate clumped isotope thermometry and enables new investigations into conditions of dolomite formation. Finally, the clumped isotope thermometer can be applied to key intervals for investigating paleoclimate and testing paleoclimate questions. To determine sea surface temperatures and water composition in Bermuda during the onset of Last Interglacial sea level rise, conventional and clumped isotope measurements of fossil shells were conducted. These results suggest meltwater influence and temperatures as much as 10° C colder than modern, requiring explanations that allow for major changes in North Atlantic surface ocean circulation. They also illustrate extreme sensitivity of Bermudian climate to broad-scale climate and ocean circulation changes.
... We use 30 °C/km for simplicity. We note that this geotherm is higher than the range given in Winkelstern and Lohmann (2016) of 13-18 °C/km for the area, which is based on unpublished geothermal measurements from the Getty Corporation (Epstein and Clark, 2009). We prefer the ODP-based geothermal gradients as the data and methodology used to calculate them are traceable and were conducted during drilling with the explicit purpose of measuring geothermal gradients (Pribnow et al., 2000). ...
The measurement of multiply isotopically substituted (‘clumped isotope') carbonate groups provides a way to reconstruct past mineral formation temperatures. However, dissolution-reprecipitation (i.e., recrystallization) reactions, which commonly occur during sedimentary burial, can alter a sample’s clumped-isotope composition such that it partially or wholly reflects deeper burial temperatures. Here we derive a quantitative model of diagenesis to explore how diagenesis alters carbonate clumped-isotope values. We apply the model to a new dataset from deep-sea sediments taken from Ocean Drilling Project site 807 in the equatorial Pacific. This dataset is used to ground truth the model. We demonstrate that the use of the model with accompanying carbonate clumped-isotope and carbonate δ18O values provides new constraints on both the diagenetic history of deep-sea settings as well as past equatorial sea-surface temperatures. Specifically, the combination of the diagenetic model and data support previous work that indicates equatorial sea-surface temperatures were warmer in the Paleogene as compared to today. We then explore whether the model is applicable to shallow-water settings commonly preserved in the rock record. Using a previously published dataset from the Bahamas, we demonstrate that the model captures the main trends of the data as a function of burial depth and thus appears applicable to a range of depositional settings.
... time scale evaluated here (Passey and Henkes, 2012;Henkes et al., 2014;Stolper and Eiler, 2015). Under no realistic geothermal scenario have samples shallower than 1500 m experienced temperatures >100 °C, and measurements suggest that this threshold is reached closer to 4 km depth (Epstein and Clark, 2009). ...
Carbonate clumped isotope paleothermometry is a promising method for paleoclimate reconstructions and understanding diagenetic processes, although clumped isotope compositions (measured as Δ47 values) can be altered by recrystallization and thermal resetting. Several studies have investigated the effects of deep burial and exhumation on Δ47 values; we present Δ47 values of samples from an ~4500-m-long drill core from Andros Island, Bahamas, which represent the spectrum from shallow to deep burial conditions. These limestones and dolomites originally formed under near-surface temperature conditions, which are reflected by ~27 °C Δ47 temperatures in samples buried to <~1.3 km depth. Below this depth, calculated Δ47 temperatures increase by ~10 °C, indicating a shift from preserved near-surface temperatures to diagenetically modified values. Importantly, this shift is not accompanied by a change in δ18O and δ13C values or cathodoluminescence, and non-uniform micrometer-scale crystallization is observed in both Δ47-altered and unaltered rocks. Similar Δ47 temperatures recorded by each carbonate phase suggest that mineral-specific differences are minor. Finegrained dolomites and calcites are viable materials for recording surface temperature conditions, and both are susceptible to alteration of their primary clumped isotope abundances during burial diagenesis. Our results demonstrate that original Δ47 values of fine-grained carbonates can be altered under mild diagenetic conditions.
... Principal geological reference for this study is a paper by Epstein and Clark (2009) who presented findings of the Getty Frontier Exploration team in the southeastern Bahamas in the 1980s. The paper describes in some detail stratigraphic information revealed in the six Bahamas wells. ...
Analyses of gravity, magnetic and seismic data in southeastern Bahamas provide
clear evidence for the presence of a Jurassic salt diaper at a depth of 5000 m. To date, salt diapirism has not yet been identified in the Bahamas. The deduced feature is adjacent to a deep basin, has a drainage area of over 9000 km2 and appears to be part of a Triassic-Jurassic syn-rift that extends 800 km southeast from Florida’s Sunniland Trend.
The purpose of this study is to document the cyanobacteria from Lucayan Archipelago, and to revise and update nomenclature to reflect current taxonomic understanding for previously documents species. There were 88 species identified in the Bahamas islands and 6 species from the Turks and Caicos Islands. There are a total of 89 species and intraspecific taxa from this archipelago. The dominant order was Nostocales with 23 species (16 Rivulariaceae , 6 Scytonemataceae , and only 1 Aphanizomenonaceae ). Oscillatoriales (45 Microcoleaceae and 5 Oscillatoriaceae ) was the second most abundant order with 22 species. Chroococcales was the third most abundant order with 20 species (8 Microcystaceae , 6 Pleurocapsaceae , 4 Chroococcaceae , and Gomphosphaeriaceae and Cyanothrichaceae only had one species each). Leptolyngbyales was the fourth most abundant order with 13 species (8 Trichocoleusaceae and 5 Leptolyngbyceae ). Coleofasciculales had 4 species (4 Coleofasciculaceae ), 2 Chroococcidiopsidales , 2 Spirulinales and three orders ( Desertifilales, Pleurocapsales , and Pseudanabaenales ) only had one species each.
The first commercial oil production in the Philippines is from the Nido “B” field located about 30 miles northwest of Palawan. Oil is being produced from a Lower Miocene atoll-like carbonate buildup at a depth of approximately 6,800 feet.
During development drilling, a 30-foot core was taken in the Nido B-3A well drilled near the margin of the buildup. Rocks in the core are intraclast packstones and skeletal packstones/grainstones. Intraclasts are poorly sorted, moderately well-rounded, and surrounded by a shaly matrix containing abundant planktic foraminifers. They were apparently derived from a marine-cemented reef rich in red algae and corals. Fragments of red algae, corals) larger benthic foraminifers and molluscs are the dominant grains in the skeletal pack-stones and grainstones. All rocks in the core were deposited as proximal fore-reef talus.
Essentially all primary porosity in these limestones was filled by marine cement now recrystallized to calcite. A small amount of equant calcite filled the remaining primary pore space. Secondary vuggy porosity is present locally, but most porosity in the core is in a complex set of anastamosing hairline fractures that divides the rock into irregular polygons a few centimeters in diameter. Good permeability in these fractures allowed the well to produce more than 10,000 BOPD after treatment with acid during initial testing.
Study of the Nido B-3A core is significant because it shows that (1) fore-reef talus may be a good reservoir facies in Miocene buildups in the Philippines, (2) hairline fractures can apparently form a significant reservoir in the subsurface even though they are barely visible in polished slabs, (3) fore-reef talus, like the reef framework, is susceptible to extensive marine cementation, (4) micritization is a minor process in fore-reef talus, and (5) difficulty would exist in trying to distinguish fore-reef talus from other facies of a reef complex in small samples, particularly cuttings and sidewall cores.
The depositional environments in which the evaporites accumulated include non-marine settings and marine settings in an epeiric sea. The products of these settings are essentially indistinguishable. Epeiric seas of the kind inferred are nowhere present in the modern world. Evaporites accumulated on large stretches of the epeiric sea floor which became subaerially exposed. Cores of undeformed salt reveal stromatolites composed of anhydrite and halite. In such a setting carbonates were prone to dolomitization. Fresh water through leaching selectively removed non-replaced calcium carbonate from dolomite creating porosity. Mat-forming algae played a dual role: 1) they provided part of the Mg for dolomite formation and 2) mat-forming algae are among the best of the source rocks. - Author
Sediments of the Gabon basin, 16,000-18,000 m thick, range in age from Early Cretaceous, or perhaps latest Jurassic, to recent. A salt layer of late Aptian age separates the almost completely continental facies of the presalt or Cocobeach sequence from the largely marine postsalt sediment. Since its inception, the basin has taken the form of a half graben whose eastern edge consisted of a series of three, or perhaps more, hinge zones which migrated successively farther west and which controlled the distribution of depositional environments and facies changes. The recognition of the hinge zones is of great importance in predicting reservoir trends. In its central part, the basin was bordered on the west by the Anguille basement high, and as the position of this high remained more or less fixed at the (present) continental margin, the basin became narrower with time. It was not until Miocene time that the Anguille high subsided strongly and ceased to influence deposition. Mainly during late Cocobeach deposition the Lambarene-Ikassa Kongo-Gamba horst zone was formed. After the peneplanation of horsts and grabens alike, the sea invaded the Gabon basin for the first time. The relatively thin, transgressive, coastal-marine sequence between the unconformity and the overlying salt is called the "Gamba formation," whose sandstones are important oil producers. The main productive trend is related to the structural configuration of the underlying horsts and grabens. The third hinge zone (Atlantic hinge belt) was active during deposition of much of the postsalt sequence. This hinge zone probably extends over the whole length of the basin and controlled the separation of deeper marine (source) environments on the west from shelf (reservoir) environments on the east. The lack of shale members to cap potential reservoir rocks along at least parts of the hinge belt may explain why no important oil accumulations have been found so far. The principal oil fields producing from the postsalt sediments are 60-100 km west of the Atlantic hinge belt, partly on the eastern slope of the Anguille basement high. The environments of deposition of the producing sediments vary from brackish-water estuarine to marine-distal deltaic; the largest oil fields are on the west not as a result of optimum reservoir conditions but because of the timely development of nonpiercing salt-induced domal structures of large areal extent. Steep salt piercements are present farther east in deeper parts of the depositional basin, and oil accumulations related to these piercements tend to be smaller.