Burial History Reconstruction Research Articles

A bulk kinetic, burial history and thermal modeling study of the Albian Kazhdumi and the Eocene-Oligocene Pabdeh formations in the Ahvaz anticline, Dezful Embayment, Iran

The Ahvaz anticline which is located in the middle region of the Dezful Embayment is certainly one of the most important oilfields in Iran. Burial history reconstruction and one-dimensional thermal maturity modeling of the Late Jurassic to Present-day successions have been conducted in the Ahvaz anticline. The petroleum generation bulk kinetic parameters of the Kazhdumi and Pabdeh formations were computed by a non-isothermal open system pyrolysis at different heating rates. Bulk kinetic experiments reveal that a relatively narrow distribution of activation energies, range from 40 to 52kcal/mol for the Kazhdumi samples and from 42 to 54kcal/mol for Pabdeh shales, are directly related to the homogeneity in the organic matter. The principal activation energies of 46kcal/mol and 48kcal/mol, frequency factors of 4.36×10+11s−1 and 1.91×10+12s−1 and hydrogen indexes (HI) of 123 and 351mg/g TOC have been computed for the Kazhdumi and Pabdeh formations, respectively. The lithology composition (percentages of shale, marl, gypsum, limestone and sandstone) of the Late Jurassic-Present formations in the selected wells was determined by using cutting samples and wireline well log data. The computed kinetic parameters and derived lithology composition of strata were applied to thermal maturity modeling. Thermal maturity modeling and burial history reconstruction which is calibrated by vitrinite reflectance data and formation temperature indicate that the palaeo-heat flow ranges from 37mW/m2 to 41mW/m2.The Kazhdumi Formation entered the main stage of oil generation during the Late Miocene (6.5Ma) and the Pabdeh Formation reached early stage of oil window from the Early Pliocene (3.5Ma). Our results provide the bulk petroleum generation kinetic parameters description of the Kazhdumi and Pabdeh formations and demonstrate the thermal maturation state of source rocks, especially the Albian shales, that has been recognized as most important source rock in the Dezful Embayment.

BURIAL, TEMPERATURE AND MATURATION HISTORY OF THE AUSTRAL AND WESTERN MALVINAS BASINS, SOUTHERN ARGENTINA, BASED ON 3D BASIN MODELLING

This paper presents a numerical petroleum systems model for the Jurassic‐Tertiary Austral (Magallanes) Basin, southern Argentina, incorporating the western part of the nearby Malvinas Basin. The modelling is based on a recently published seismo‐stratigraphic interpretation and resulting depth and thickness maps. Measured vitrinite reflectance data from 25 wells in the Austral and Malvinas Basins were used for thermal model calibration; eight calibration data sets are presented for the Austral Basin and four for the Malvinas Basin. Burial history reconstruction allowed eroded thicknesses to be estimated and palaeo heat‐flow values to be determined. Six modelled burial, temperature and maturation histories are shown for well locations in the onshore Austral Basin and the western Malvinas Basin. These modelled histories, combined with kinetic data measured for a sample from the Lower Cretaceous Springhill Formation, were used to model hydrocarbon generation in the study area. Maps of thermal maturity and transformation ratio for the three main source rocks (the Springhill, Inoceramus and Lower Margas Verdes Formations) were compiled. The modelling results suggest that deepest burial occurred during the Miocene followed by a phase of uplift and erosion. However, an Eocene phase of deep burial leading to maximum temperatures cannot be excluded based on vitrinite reflectance and numerical modelling results. Relatively little post‐Miocene uplift and erosion (approx. 50–100 m) occurred in the Malvinas Basin.Based on the burial‐ and thermal histories, initial hydrocarbon generation is interpreted to have taken place in the Early Cretaceous in the Austral Basin and to have continued until the Miocene. A similar pattern is predicted for the western Malvinas Basin, with an early phase of hydrocarbon generation during the Late Cretaceous and a later phase during the Miocene. However, source rock maturity (as well as the transformation ratio) remained low in the Malvinas Basin, only just reaching the oil window. Higher maturities are modelled for the deeper parts of the Austral Basin, where greater subsidence and deeper burial occurred.

Genetic origin of sour gas condensates in the Paleozoic dolomite reservoirs of the Tazhong Uplift, Tarim Basin

Twenty gas condensates, six oils and their associated formation water from the Tazhong Uplift in the Tarim Basin, NW China were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates. Thermochemical sulfate reduction (TSR) is the main secondary alteration process relevant to the generation of sour gas condensates. Those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations. The hydrogen sulfide (H2S) concentrations in gas phase show a strong positive correlation to gas/oil ratio, organosulfur compound (OSC) concentrations in the liquid phase. While methane isotopic values are mainly controlled by thermal maturity, wet gas components show unusual 13C enrichment at a low degree of thermal cracking. Geochemical anomalies occur mainly in the Lower Ordovician Yingshan Fm. where more H2S has been discovered, while the Upper Ordovician Lianglitage Fm. shows less irregularity and H2S concentration is much lower. Generation of sour gas condensate is most likely caused by TSR alteration, which could oxidize liquid hydrocarbons and form a large quantity of OSCs. Isotopic enrichment of 34S in formation water sulfate and elevated OSC concentrations provide supplemental evidence of TSR alteration. Both burial history reconstructions based on fluid inclusion analysis and PVT simulation indicate that in situ TSR reactions mainly occurred around last 10 Ma during the late Himalayan orogeny and overall TSR extent is relatively low in the Tazhong Uplift.

Multiple-stage migration and accumulation of Permian lacustrine mixed oils in the central Junggar Basin (NW China)

The Permian lacustrine petroleum system in the central Junggar Basin, northwest China, has recently obtained exploration success. These accumulations are characterized by mixed oils and multiple-stage migration and accumulation. This process, however, has not been well constrained, limiting the understanding of the general laws of hydrocarbon migration and accumulation and making it difficult to define exploration strategies. Here, we reconstruct this process mainly based on oil-bearing fluid inclusion analyses including petrography, homogenization temperature and sequential extraction. Inclusion petrography shows that petroleum inclusions dominantly display yellow fluorescence, while intergranular free oils mostly display strong blue green fluorescence. This implies that the reservoirs seem to have experienced at least two stages of hydrocarbon charge from different sources and/or maturities. The sources/maturities are determined through reservoir sequential extraction analyses. The free oils are mixed and sourced from the Lower Permian Fengcheng Formation (P1f) and the Middle Permian Lower Wuerhe Formation (P2w), while the inclusion oils are also mixed and derived mainly from the P1f source. Both oils are characterized by moderate maturity. Homogenization temperature measurements of oil-bearing fluid inclusions, combined with reconstruction of reservoir burial and thermal history, not only further verify multiple hydrocarbon charging events but also characterize the timing, which indicates a three-stage petroleum charging model. During the first stage of the Late Triassic, Carboniferous reservoirs were charged by P1f-sourced oils, which were dysmigrated or remigrated to Jurassic reservoirs during the second stage (i.e., the Early Cretaceous). In contrast, the third stage from the Late Cretaceous represents another hydrocarbon charge event that is the primary oil accumulation sourced from the P2w rocks. According to this model, both primary and secondary oil accumulations contribute to the current region's exploration success, and provide future exploration targets.

Origin and Filling Model of Natural Gas in Jiannan Gas Field, Sichuan Basin, China

The chemical and stable isotopic compositions of natural gases from Jiannan Gas Field, Sichuan Basin, China, were investigated to elucidate the origin of the gas and to reconstruct the reservoir filling history. The natural gases in Jiannan Gas Field are dominated by CH4 with low amounts of non-hydrocarbon (e.g. CO2, H2S and N2) components. The stable carbon isotope compositions of methane and its homologues vary widely, with an isotope trend of δ13C1≥ δ13C2< δ13C3. Structural restoration and burial history reconstruction indicated contrasting gas-filling histories for the gas reservoirs. Natural gas in the Carboniferous reservoirs was sourced from oil and gas cracking and thermal decomposition of kerogen in the lower Silurian shale and sealed by Lower Permian shale. Natural gas in the Upper Permian reservoir was derived primarily from Upper Permian mudstone with a minor contribution from Lower Silurian shale. The sulfur-bearing gas was probably sourced from Permian marine source rocks and migrated into the Lower Triassic reservoirs along unconformities and faults. Long-distance vertial migration may have caused the loss of H2S.

Numerical approach for thermal history modelling in multi-episodic rifting basins

Pre-existing models for thermal history modelling have shown deficiency in explicit algorithms to establish the quantitative relationship between maturity indices and thermal gradients in some sedimentary basins that experienced multi-episodic rifting evolution. In this study, a forward and inverse combination model (FICM) is proposed to estimate the vitrinite reflectance (Ro) and thermal gradients. The forward module is used to calculate Ro values. It couples the EASY%Ro model with burial history reconstruction with consideration of thermal gradient variations during basin evolution. The inverse module reconstructs histoical thermal gradients by calibrating cmputed Ro against measured Ro data. The time-temperature series is a necessary input for both forward and inverse modules. Sample density is a profound factor influencing the accuracy of modelling results. In order to obtain satisfying outputs, a sufficient sample density is required. Thermal gradients are assumed to vary linearly between two given samples. Modelling results of case studies indicate that the sensitivity of heating time to Ro evlution is differnt with thermal gradients depending on geolgoical setting. Three difffernt districts, which include the time-sensitive area, the temperature-sensitive area and the non-sensitive area, can be recognized on the the relationship map among Ro variations, heating time and geothermal gradients. This model can be applied to reconstruct the thermal history and maturation evolution in a basin that has undergone complex multi-episodic rifting.

Constraining Cenozoic exhumation in the Faroe‐Shetland region using sonic transit time data

AbstractThe Mesozoic‐Cenozoic basins located between the Faroe, Orkney and Shetland Islands along the NE Atlantic Margin are actively explored oil and gas provinces whose subsidence histories are complicated by multiple tectonic factors, including magmatism, inversion and regional‐scale uplift and tilting, that have resulted in spatially variable exhumation. These basins also exhibit nonburial related, transient Cenozoic heating anomalies that make thermal history interpretation and burial history reconstructions problematic. In this study, we have applied a compaction‐based approach, which is less susceptible to distortions from transient heating, to provide new constraints on Cenozoic burial and exhumation magnitudes in the UK sector of the Faroe‐Shetland region using sonic transit time data from Upper Cretaceous marine shales of the Shetland Group in 37 wells. As estimates of exhumation magnitude depend critically on the form of the normal sonic transit time‐depth trend, a new marine shale baseline trend was firstly constructed from shales presently at maximum burial, consistent with other marine shale baseline trends of different ages from nearby basins. Our results indicate that Upper Cretaceous marine shales are presently at or near (i.e. within ≤100 m net exhumation) maximum burial depths in the Møre and Magnus basins in the northeast of the study area as well as in the deeper water Faroe‐Shetland Basin (i.e. Flett and Foula sub‐basins). However, Upper Cretaceous strata penetrated by wells in the southwest have been more deeply buried, with the difference between maximum burial depth and present‐day values (net exhumation) increasing from ca. 200 to 350 m along the central and northeastern parts of the Rona High to ca. 400–1000 m for wells located in the West Shetland Basin, North Rona Basin and southwestern parts of the Rona High. Although the precise timing of exhumation is difficult to constrain due to the complex syn‐ to post‐rift tectonostratigraphic history of vertical movements within the Faroe–Shetland region, our estimates of missing section, together with available thermal history constraints and seismic‐stratigraphic evidence, implies that maximum burial and subsequent exhumation most likely occurred during an Oligocene to Mid‐Miocene tectonic phase. This was probably in response to major post‐breakup tectonic reshaping of this segment of the NE Atlantic Margin linked to a coeval and significant reorganization of the northern North Atlantic spreading system, suggesting that fluctuations in intraplate stress magnitude and orientation governed by the dynamics of plate‐boundary forces exert a major control on the spatial and temporal variations in differential movements along complexly structured continental margin.

Burial history reconstruction and thermal maturity modeling for the Middle Cretaceous–Early Miocene Petroleum System, southern Dezful Embayment, SW Iran

The Middle Cretaceous–Early Miocene Petroleum System is the most important hydrocarbon system in the Zagros Basin. Rock-Eval pyrolysis, organic petrography and thermal 1D modeling were performed on wells located in the southern Dezful Embayment. The results provide information on organic matter quantity and quality as well as burial and thermal histories of source rocks in the framework of the basin evolution. A total of 98 cutting samples from BK-4, KK-1, SI-4 and RS-2 wells were pyrolyzed by Rock-Eval 6. Also thermal maturity of organic matter derived from Tmax of Rock-Eval was interpreted in conjunction with vitrinite reflectance data (50 samples). The TOC values of the sequences mainly range from 0.5 to 2.5wt.%. The average values of Tmax and vitrinite reflectance indicate that samples from the wells have reached maturities corresponding to early/mid oil generation. Reconstruction of thermal history suggests that various steady heat flow values (62, 54, 50 and 56mW/m2 for BK-4, KK-1, SI-4 and RS-2 wells, respectively) resulted in the best fit between the calculated and the observed vitrinite reflectance and bottom hole temperatures in our modeling. Thus, values of heat flow decrease from 60mW/m2 in the southern basin margin to about 50mW/m2 along the NE of the Dezful Embayment and the Zagros Basin. According to our modeling results, the onset of oil generation from source rocks in the studied area occurs well after deposition of the seal rock and formation of traps, ensuring entrapment and preservation of migrated hydrocarbons.

Reconstruction of burial history, temperature, source rock maturity and hydrocarbon generation in the northwestern Dutch offshore

Abstract3D basin modelling is used to investigate the history of maturation and hydrocarbon generation on the main platforms in the northwestern part of the offshore area of the Netherlands. The study area covers the Cleaverbank and Elbow Spit Platforms. Recently compiled maps and data are used to build the input geological model. An updated and refined palaeo water depth curve and newly refined sediment water interface temperatures (SWIT) are used in the simulation. Basal heat flow is calculated using tectonic models. Two main source rock intervals are defined in the model, Westphalian coal seams and pre-Westphalian shales, which include Namurian and Dinantian successions. The modelling shows that the pre-Westphalian source rocks entered the hydrocarbon generation window in the Late Carboniferous. In the southern and central parts of the study area, the Namurian started producing gas in the Permian. In the north, the Dinantian source rocks appear to be immature. Lower Westphalian sediments started generating gas during the Upper Triassic. Gas generation from Westphalian coal seams increased during the Paleogene and continues in present-day. This late generation of gas from Westphalian coal seams is a likely source for gas accumulations in the area.Westphalian coals might have produced early nitrogen prior to or during the main gas generation occurrence in the Paleogene. Namurian shales may be a source of late nitrogen after reaching maximum gas generating phase in the Triassic. Temperatures reached during the Mid Jurassic were sufficiently high to allow the release of non-organic nitrogen from Namurian shales.

Paleo-heat flow evolution of the Tabei Uplift in Tarim Basin, northwest China

The paleo-heat flow evolution of the Tabei Uplift in the Tarim Basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling. Numerous geological parameters, such as, temperature data and missing sediment thickness by erosion were used in the modeling. The basin model was calibrated using 460 measured vitrinite reflectance (% VR o) and vitrinite-like maceral reflectance (% VLMR o) values to constrain the validity of the maturity model. The heat flow history of the Tabei Uplift, Tarim Basin shows the following characteristics: (1) the highest paleo-heat flow was predicted to have occurred in the Early Ordovician as 65 ± 5 mW/m 2, and gradually decreased to 55 ± 5 mW/m 2 during the Late Carboniferous; (2) a thermal kick was modeled to have occurred in the Permian as suggested by an abrupt rise in the heat flow; (3) the heat flow gradually decreased since the Triassic; (4) the present day heat flow was predicted to be as low as 38 mW/m 2. This heat flow history honors the geologic and tectonic evolution history of the Tabei Uplift and is suggested as the best case heat flow model.

SOURCE ROCK POTENTIAL OF THE BLUE NILE (ABAY) BASIN, ETHIOPIA

The Blue Nile Basin, a Late Palaeozoic ‐ Mesozoic NW‐SE trending rift basin in central Ethiopia, is filled by up to 3000 m of marine deposits (carbonates, evaporites, black shales and mudstones) and continental siliciclastics. Within this fill, perhaps the most significant source rock potential is associated with the Oxfordian‐Kimmeridgian Upper Hamanlei (Antalo) Limestone Formation which has a TOC of up to 7%. Pyrolysis data indicate that black shales and mudstones in this formation have HI and S2 values up to 613 mgHC/gCorg and 37.4 gHC/kg, respectively. In the Dejen‐Gohatsion area in the centre of the basin, these black shales and mudstones are immature for the generation of oil due to insufficient burial. However, in the Were Ilu area in the NE of the basin, the formation is locally buried to depths of more than 1,500 m beneath Cretaceous sedimentary rocks and Tertiary volcanics. Production index, Tmax, hydrogen index and vitrinite reflectance measurements for shale and mudstone samples from this areas indicate that they are mature for oil generation. Burial history reconstruction and Lopatin modelling indicate that hydrocarbons have been generated in this area from 10Ma to the present day.The presence of an oil seepage at Were Ilu points to the presence of an active petroleum system. Seepage oil samples were analysed using gas chromatography and results indicate that source rock OM was dominated by marine material with some land‐derived organic matter. The Pr/Ph ratio of the seepage oil is less than 1, suggesting a marine depositional environment. n‐alkanes are absent but steranes and triterpanes are present; pentacyclic triterpanes are more abundant than steranes. The black shales and mudstones of the Upper Hamanlei Limestone Formation are inferred to be the source of the seepage oil.Of other formations whose source rock potential was investigated, a sample of the Permian Karroo Group shale was found to be overmature for oil generation; whereas algal‐laminated gypsum samples from the Middle Hamanlei Limestone Formation were organic lean and had little source potential

Reconstruction of burial history of eroded Mesozoic strata using kimberlite shale xenoliths, volcaniclastic and crater facies, Northwest Territories, Canada

Reconstruction of Mesozoic and Cenozoic sedimentary ‘cover’ on the Precambrian shield in the Lac de Gras diamond field, Northwest Territories, Canada, has been achieved using Cretaceous and early Tertiary sedimentary xenoliths and contemporaneous organic matter preserved in volcaniclastic sediments associated with late Cretaceous to early Tertiary kimberlite pipe intrusions, and in situ, Eocene crater lake, lacustrine and peat bog strata. Percent reflectance in oil (%Ro) of vitrinite within shale xenoliths for: (i) Albian to mid-Cenomanian to Turonian ranges from > 0.27 to 0.42 %Ro (mean = 0.38 %Ro), (ii) Maastrichtian to early Paleocene from 0.24 to < 0.30%; (iii) latest Paleocene to early middle Eocene 0.15 to < 0.23 %Ro (mean = 0.18 %Ro). These levels of thermal maturity are corroborated by Rock Eval pyrolysis Tmax (°C) and VIS region fluorescence of liptinites, with wavelengths of maximum emission for sporinite, prasinophyte alginite and dinoflagellates consistent with vitrinite reflectance of 0.20 to < 0.50 %Ro. Burial–thermal history modeling, constrained by measured vitrinite reflectance and porosity of shale xenoliths, predicts a maximum burial temperature for Mid to Late Albian strata (∼115 Ma) of 60 °C with ∼1.2 to 1.4 km of Cretaceous strata in the Lac de Gras kimberlite field region prior to major uplift and erosion, which began at 90 Ma. Late Paleocene to middle Eocene volcanic crater lake lacustrine to peat bog strata were only buried to a few hundreds of meters and are in a peat-brown coal stage of thermal maturation.

Geochemistry and petrology of a Middle Tithonian limestone-marl rhythmite in the Neuquén Basin, Argentina: depositional and burial history

Abstract The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), is lithologically and geochemically similar to limestone-marl alternations from the Late Jurassic of the northern hemisphere. Both marls and limestones are pelbiomicrites and biopelmicrites principally composed of pellets, radiolaria, forams, ostracods, equinoids, spicules of sponges and gastropods, cemented by several generations of calcite cement. Smectite and interlayers are the major epiclastic components of the fraction below 2 μm, reflecting pedogenic processes developed on volcanogenic source rocks. More abundant kaolinite in some marls reflects stronger humid conditions in the source area and enhanced terrigenous supply. Al 2 O 3 content is demonstrated to be a reliable indicator of clastic input. The same is not true for silica, often related to high biogenic productivity of siliceous organisms. Rocks show total organic carbon (TOC) contents up to 1.95% and constitute regular to good sources for hydrocarbons, although thermally immature. Rhythmites formed gently sloping mounds accumulated in a distal submarine ramp under low-energy and poorly oxygenated open-sea conditions. Sedimentation rates were high due to high productivity on the sea surface, and supply of terrigenous and carbonate sediments transported by suspension plumes originated in shallow, photic areas. δ 13 C values correspond well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ 13 C point to a productivity crisis or a mixture of water layers in a stratified sea with a periodicity of 400 ka, which could be a result of changes in the orbital eccentricity of the Earth. Light isotopic composition of O in bulk rocks is the result of diagenetic neomorphism and cement precipitation. Calculated palaeotemperatures from δ 18 C are consistent with those derived from measured vitrinite reflectance ( Ro% ) and burial history reconstruction. Data indicate initial burial during the Tithonian extending up to the Lower Cretaceous, a short period of uplift (Intravalaginian tectonic phase), and renewed uplift during the Cenomanian followed by significant Late Cretaceous sedimentation and Pliocene thrusting.

Burial and thermal history of the West Bashkirian sedimentary basins

The GALO system is applied to the numerical reconstruction of burial and thermal histories of the West Bashkirian lithosphere from the Riphean to the present. An analysis of the variation in tectonic subsidence of the basin during its development is utilized to estimate approximately the mantle heat flow variations. Our variant of basin evolution suggests that after cooling in the Early Riphean, the rather weak thermal reactivations have not led to considerable heating of the lithosphere in the study region. Surface heat flow decreased from relatively high values in the Early Riphean (60–70 mW/m 2 in the eastern area and 40–50 mW/m 2 in the western part) to present-day values of 32–40 mW/m 2. In spite of the relatively low temperature regime of the basin as a whole, a syn-rifting deposition of more than 10 km of limestone, shale and sandstone in the Riphean resulted in rather high temperatures (180–190 °C) at the base of present-day sedimentary blanket in the eastern area. In agreement with the observed data, computed present-day heat flow through the sediment surface increases slightly from 32 to 34 mW/m 2 near the west boundary of the region to 42 mW/m 2 near the boundary of the Ural Foldbelt, whereas the heat flow through the basement surface decreases slightly from 28–32 to 24–26 mW/m 2 in the same direction. The mantle heat flow is only 11.3–12.7 mW/m 2, which is considerable lower than mean heat flow of the Russian Platform (16–18 mW/m 2) and comparable with the low heat flow of Precambrian shields.

Reconstruction of Late Paleozoic heat flows and burial histories at the Rhenohercynian-Subvariscan boundary, Germany

The thermal and burial history of the Herzkamp syncline, located in the transition zone between the Variscan Rhenish Massif and the Ruhr foreland basin (western Germany), was reconstructed using PDI/PC-1D-basin modelling software (IES). The models were calibrated with new vitrinite reflectance data measured on Palaeozoic outcrop samples. High sample density and quality of the calibration data allowed a 3D reconstruction of the heat flow as well as of burial and erosion history. Vitrinite reflectance values range from 0.8 to 4.9%Rr and generally increase with increasing stratigraphic age. The coalification pattern confirms pre-tectonic maturation, especially in the western part of the study area. A "low-coalification zone" showing stagnating/decreasing coalification with increasing stratigraphic age exists, however, northeast of the Ennepe thrust, indicating synorogenic coalification. This anomaly is explained by early thrusting in the northern Rhenish Massif resulting in restricted burial/early uplift and thus lower thermal maturity. One result of numerical modelling is that palaeo-heat flows during maximum burial (Westphalian or post-Westphalian) decreased southwards from approximately 65 to less than 50 mW/m2. Maximum burial depths for the base and top of the Namurian also decrease southwards from 7000 to 3600 m and 4600 to 1800 m, respectively, resulting in southwards-decreasing coalification of the respective stratigraphic horizon. Eroded overburden increases southwards (3100–5700 m), with the exception of the low-coalification zone, which is characterised by lower amounts of eroded overburden (1300–2900 m) and an earlier onset of erosion, i.e. in the Westphalian B rather than Westphalian D or post-Westphalian.

New insights on petroleum migration from the application of 4D basin modelling in oil and gas exploration

Basin modelling is a valuable tool for investigating the hydrocarbon system in the sub-surface, and is widely used as an aid in reducing exploration risk. Application of the technology in oil and gas exploration imposes a rigorous, systematic approach to prospect evaluation and provides new insights on the processes of hydrocarbon migration, including flow rates, migration distances and charging histories. The ability to build models quickly and run multiple simulations, particularly where data are scarce, allows the explorationist to pose ‘What if?’ questions to investigate the sensitivity of the petroleum system to critical parameters, such as source rock properties. Reconstruction of 3D burial history, structural restoration, dynamic fault properties and seamless integration with seismic data are defined as some of the important unknown factors in basin modelling and the main limitations to its more routine application in petroleum exploration. New developments should provide a balance between geological sophistication of the model and the time required to run simulations.

Burial, Maturation, and Petroleum Generation History of the Arkoma Basin and Ouachita Foldbelt, Oklahoma and Arkansas

Removed overburden, burial, maturation, and petroleum generation analysis indicates that maturity in the Arkoma Basin and the Ouachita Foldbelt is explained effectively using simple burial models that account for the significant surface erosion that has occurred and assuming geothermal gradients similar to present-day gradients have been approximately constant through geologic time. Regional models, based on analysis at 115 well locations, indicate that from 5,000 to 15,000 ft (1.5–4.5 km) of section, differing with location from north to south and west to east, has been removed from the Arkoma Basin region, and as much as 25,000–40,000 ft (7.5–12 km) have been removed from areas of the Ouachita Foldbelt. Based on burial and thermal history reconstruction, increasing maturation from west to east across the basin is primarily the result of increasing overburden and subsequent surface erosion from west to east. The models predict most publicly available vitrinite reflectance data within a factor of 1.5 at two standard deviations. Comparison of model and measured reflectance-depth trends in six wells indicates that hydrothermal fluid movement should not have modified reflectance by more than approximately 20% in the center of the basin. Analysis indicates that most of the basin is overmature for oil production from intervals below the Spiro Sandstone, except to the north and northwest. Although thermal maturities are high, methane is stable throughout the basin. Except for the basal Arbuckle Group, all formations were thermally immature for oil generation prior to burial by the Mississippian and Morrowan in the Ouachita Foldbelt of Oklahoma and by the Atokan and Desmoinesian over most of the basin and study area. In the deeper part of the present basin, all strata entered and passed through the oil window during or within 10 My after Atokan time. Because no additional major quantities of hydrocarbons were generated after Atokan time, the hydrocarbons must have been emplaced and trapped during this brief time interval.

A review of techniques used to determine geological and thermal history in the Southern North Sea

Abstract The Southern North Sea has been a prolific source of gas to the UK economy over the last 30 years. As many of the producing fields are now becoming depleted, attention has switched to exploration in more marginal areas of the basin. In order to evaluate suitable structures in these areas, an understanding of the complex structural and thermal history of the Southern North Sea Basin is required. Particular emphasis must be placed upon the prediction of source rock maturity, the timing of generation and migration of hydrocarbons, and the thermal controls exerted on potential reservoir sand units. Over large parts of the basin the Carboniferous source rocks are not at their maximum depth of burial at present day. A variety of techniques utilizing sonic velocity, vitrinite reflectance and apatite fission track analysis have been used to quantify the magnitude of uplift that the Carboniferous section has undergone through geological time. With the exception of apatite fission track analysis, these techniques provide only an estimate of ‘net uplift’, i.e. the amount of uplift relative to maximum depth of burial or maximum temperature. They record neither the timing and effects of individual uplift events nor the timing of maximum burial prior to net uplift. In order to accurately reconstruct the burial and thermal history of the basin these techniques must be integrated with a detailed knowledge of the geological history of the Southern North Sea. The use of these techniques in isolation, together with a lack of appreciation of the limitations of the data has in the past resulted in spurious results, leading to criticism and mistrust of the techniques. This paper attempts to show that when correctly applied and in appropriate geological circumstances, each analytical technique can provide useful data for burial and thermal history reconstruction. Examples are shown to highlight the effects of incomplete data interpretation and the impact of inaccurate determination of present day temperatures. Particular attention is paid to determining the burial and thermal history of the economically important Carboniferous source and Permian reservoir units, and the interpretation of uplift data is restricted to the post-Zechstein section.

A kinetic calculation method of homohopanoid maturation: Applications in the reconstruction of burial histories of sedimentary basins

Hydrous pyrolysis, performed at relatively low temperatures, of an immature marl yielded artificial maturation data on changes in homohopane isomer compositions. At higher temperatures enhanced amounts of 17α,21β(H)- and 17β,21α-hopanes over 17β,21β(H)-hopanes are observed. The amounts of 22S epimers increase relative to those of their 22R counterparts with increasing temperature. These experimental data, as well as sedimentary data on homohopane concentrations, have been reproduced using a kinetic calculation method, based on the thermodynamic stabilities determined by molecular mechanics calculation, using a full isomerization scheme rather than a single direct chiral isomerization step. We need to caution, however, that agreement between the predictions from a theoretical scheme and quantitative measurements is not an unequivocal proof for the chosen kinetic model. Good reproduction of the maturation data was obtained even for those obtained at the highest pyrolysis temperatures, at which significant destruction of hopane isomers was apparent. These results indicate that the kinetic calculation method can be used to model diagenetic and catagenetic reactions affecting biomarker ratios in sediments. Further computer simulations indicate that hopane-hopane interconversion reactions probably only have a minor influence on hopane isomer ratios.

Diagenesis as Related to Burial History, Pressure Regimes, and Low-Permeability Zones in the Upper Cretaceous Lower Tuscaloosa Formation, Mississippi and Louisiana: ABSTRACT

Sandstone reservoirs in the lower part of the Upper Cretaceous Tuscaloosa Formation in the Gulf Coast contain large petroleum and natural gas reserves and many are presently overpressured at depths greater than 3,350 to 4,900 m. A petrologic study was initiated to determine the types of diagenetic seals present and the timing of cementation. Hydrocarbons stain most authigenic phases including early iron-rich chlorite and quartz overgrowths and later pore-filling kaolinite and albite, and they are present in dissolution vugs in both ferroan and nonferroan carbonate cements from depths of 2,800 to 6,300 m. In sandstones from 4,575 to 6,300 m depths, two-phase hydrocarbon and hydrous inclusions are abundant between detrital quartz grains and their overgrowths and are common within the overgrowths. Uncorrected (because of included petroleum and (or) methane ) homogenization temperatures of these inclusions average 125[degrees]C, indicating that they formed in the late oil or early gas window. Present bottom hole temperatures of wells in the lower Tuscaloosa at depths of about 4,600 m average 145[degrees]C and temperatures at depths of about 5,600 m average 170[degrees]C. These values indicate that the quartz overgrowths formed before maximum burial of the Tuscaloosa. According to published hydrologic models, the Tuscaloosa was undermore » a hydrostatic pressure regime through the early Tertiary. Burial history reconstructions indicate that shales in the Tuscaloosa in central Louisiana reached the gas generation window (about 125[degrees]C) at a depth of 4 km in the Miocene. At this time, much of the lower Tuscaloosa in the downdip gas trend (greater than approximately 4,300 m) became overpressured and silica and hydrocarbon (pyrobitumen ) cements created low-permeability zones in sandstones, preserves overpressured compartments and trapping hydrocarbons.« less