Oil Seeps Research Articles

Effect of wettability on oil and water distribution and production performance in a tight sandstone reservoir

Wettability is an important factor that dominates the micro-distribution of initial oil and water, residual oil, and subsequent oil recovery in a tight oil reservoir. In this paper, integrated experimental techniques have been developed to examine the effect of wettability on oil and water micro-distribution together with production performance in a tight reservoir. Wettability alteration indicated by contact angles during the aging process was firstly monitored and measured continuously. Then, nuclear magnetic resonance (NMR) measurements were integrated with rate-controlled mercury injection (RMI) tests to determine pore and throat size distribution. Subsequently, NMR measurements were performed to not only characterize the oil and water distribution in the process of wettability change, but also evaluate the production performance and recovery potential. Furthermore, displacement experiments were carried out to visually observe the microscopic distribution of residual oil under different wettability conditions. For a strong water-wetting rock, oil is separated from the surface of a pore/throat on which a thin water film is adsorbed, and then fills the center of the pore in the form of oil droplets. The displacement experiments show that water saturation of a two-phase flow zone on relative permeability curves ranges from 22.5% to 55.1% and that water saturation at the isotonic point is 43.6% with its waterflooding oil recovery of 42.1%. For a weak oil-wetting rock, oil is mainly distributed in porous media in the form of an oil film. The water saturation of a two-phase flow zone ranges from 22.3% to 48.1%, while that of the isotonic point is 38.1% with waterflooding oil recovery of 33.2%. With wettability altering from strong water-wetting to weak oil-wetting, oil either is adsorbed on the pore and throat surface or migrates from the center of a larger pore to a smaller pore and/or throat. It is more unfavorable to the seepage of oil due to the fact that the two-phase flow zone is narrowed and a decrease in water saturation at the isotonic point, leading to a lower waterflooding oil recovery. After waterflooding, oil droplets caused by the Jamin effect, oil clusters resulted from heterogeneity, and small oil droplets dispersed at the corner of a pore are visually observed in cores with strong water-wetting. Within weak oil-wetting cores, residual oil is mainly distributed in a membrane-like form due to adsorption and oil clusters resulted from heterogeneity.

Peningkatan Kualitas Produk UKM “Peyek Bunder” Melalui Introduksi Teknologi Tepat Guna Spinner

This community service activity aims to improve the quality of Peyek Bunder by introducing a spinner in the production process of small micro enterprises (SMEs) partnered with "Peyek Bunder". The method of implementing this community service includes socialization, introduction, and mentoring. The evaluation instrument of this activity uses the before and after comparison method and the evaluation is analyzed descriptively. The results of service activities in partner SMEs show that the quality of Peyek Bunder has increased. The oil content in Peyek Bunder has decreased, which can be seen from the condition at the bottom of the packaging that there is no oil seepage. This condition is due to a better draining process as it uses appropriate technology, namely the spinner. The draining process with a spinner takes about twenty minutes minutes for a capacity of 2 kilograms of Peyek Bunder. The results obtained are crisper and the durability is one month longer than without using a spinner.

Geochemical Significance of Bicyclic Sesquiterpanes in Hydrothermal Oil of the Uzon Volcano Caldera (Kamchatka)

The composition and relative content of C14-C16 bicyclic sesquiterpanes in oil seeps from thermal springs of the Uzon Caldera have been studied by GC-MS technique. It is shown that C16 homodrimanes prevail in oil composition, C15 drimane content is insignificant, and C14 nordrimanes are present in trace amounts. Homodrimane with biological configuration 8α(H) was detected. Bicyclic sesquiterpanes of Uzon oil are probably its derivatives. It has been revealed that the relative content of C15 drimanes increases with the oil maturity. A sesquiterpane maturity index, which has not been used previously in organic geochemistry, is proposed.

High abundance of hydrocarbon-degrading Alcanivorax in plumes of hydrothermally active volcanoes in the South Pacific Ocean

Species within the genus Alcanivorax are well known hydrocarbon-degraders that propagate quickly in oil spills and natural oil seepage. They are also inhabitants of the deep-sea and have been found in several hydrothermal plumes. However, an in-depth analysis of deep-sea Alcanivorax is currently lacking. In this study, we used multiple culture-independent techniques to analyze the microbial community composition of hydrothermal plumes in the Northern Tonga arc and Northeastern Lau Basin focusing on the autecology of Alcanivorax. The hydrothermal vents feeding the plumes are hosted in an arc volcano (Niua), a rear-arc caldera (Niuatahi) and the Northeast Lau Spreading Centre (Maka). Fluorescence in situ hybridization revealed that Alcanivorax dominated the community at two sites (1210–1565 mbsl), reaching up to 48% relative abundance (3.5 × 104 cells/ml). Through 16S rRNA gene and metagenome analyses, we identified that this pattern was driven by two Alcanivorax species in the plumes of Niuatahi and Maka. Despite no indication for hydrocarbon presence in the plumes of these areas, a high expression of genes involved in hydrocarbon-degradation was observed. We hypothesize that the high abundance and gene expression of Alcanivorax is likely due to yet undiscovered hydrocarbon seepage from the seafloor, potentially resulting from recent volcanic activity in the area. Chain-length and complexity of hydrocarbons, and water depth could be driving niche partitioning in Alcanivorax.

Microbial Electrochemical-based Biofuel Cell System for Remediation of Oil Contaminated Soil

With the growing problem of oil seepage from gas stations around the world, remediation of oil-contaminated soils is receiving increased attention. Microbial electrochemical techniques have been shown to remove hydrocarbons from soil. Microbial fuel cells (MFC) show a strong potential to immobilize and dissipate contaminants using microorganisms. In this context, this review briefly introduces the conventional methods for remediation of petroleum soil contamination, focusing on the effects of different factors and configurations on the effectiveness of MFC remediation. Compared to traditional remediation methods, applying MFC to petroleum leaks in gas stations has better environmental and economic benefits. It does not require the application of chemicals or ex situ remediation of the soil, which largely reduces the cost and does not cause secondary pollution to the surrounding environment such as soil or atmosphere. In the long term, this technology has a good potential to enhance the remediation effect by changing the applied electric field, soil texture, and petroleum degrading bacteria. In addition, making soil MFC simultaneously achieve combined remediation of petroleum hydrocarbons and other contaminants remains to be studied. Finally, this paper emphasizes that there are few practical applications about MFC site remediation and there is a need to conduct site tests with large scale. If the pilot-scale tests are similar to the laboratory-scale treatment results, the technology will gain more popularity.

Magnetically controlled super-wetting surface switching between ultra-low and ultra-high droplet adhesion

Controlled liquid adhesion behaviors on super-wetting surfaces have gained significant attention, however, precise controlling of adhesion behavior is extremely hard to realize due to some inherent limitation in surface chemistry or morphology. Herein, we report a silicone-oil-infused magnetic porous polydimethylsiloxane (SMPP) surface with magnetically tunable super-wetting behaviors. Benefiting from the magnetic-field-responsive structure and interconnected porous characteristics, the SMPP can use magnetic field to control the seepage and absorption of silicone oil in the micropore channels, thereby precisely changing the adhesion of the water droplets on the surface. Compared to conventional super-wetting surfaces, this SMPP surface displays reversible in-situ conversion from ultra-high to ultra-low droplet adhesion with sliding angles ranging from 3° to 180° that hold up well even after severe cutting or abrasion. We also demonstrated that this tunable adhesion behavior can be used not only to enhance droplet manipulation in spatiotemporal but also to design unique microfluidic microreactors and antigravity transport devices. We envision that this proof-in-concept surface can serve as an important component of integrated control systems for intelligent manipulators, biochemical reactions, fluid transport, and biomedical analysis.

Oil to Source Rock Correlation of Besuki Area and its Role in Petroleum System of Banyumas Basin

Banyumas Basin is a basin that has signs of the presence of hydrocarbons, including oil and gasseep on the surface, which indicates active petroleum systems in the subsurface. A lot of oil and gas seeps werefound in Banyumas Basin, including oil seeps in Cipari and Besuki, while gas seeps were found in Mount Wetan,Karanglewas, and also Cipari. Exploration drilling wells have also been carried out, such as the Jati-1 Well, KarangNangka-1, Karang Gedang-1, Tjipari-1, and Mount Wetan-1, but there still has been no significant evidenceof this basin producing hydrocarbons. The TOC value of the outcrop samples taken in Cipari and Besuki have badvalues, while the crude oil has reflected a severe biodegradation process. Both oil seep samples and one extractbitumen contained high bicadinane (R (resin) annotation) and oleanane indicating both oils and extract to have aninput to Tertiary-sourced oils throughout Southeast Asia. Both oil seep samples do not exhibit similarity with theextract source rock. Based on this negative correlation there may be another source rocks in Banyumas Basin, derivedfrom rocks that are not only from Pemali Formation. The existence of this research is expected to be able toadd geological data, especially hydrocarbon geochemical data in Banyumas Basin, so that it can provide evidenceof the prospect and calculation of reserves contained in this basin, and an understanding of the source rock it self.

Determining the Characteristics of Natural Oil Seeps and Their Underwater Sources Based on Remote Sensing Data

Determining the Characteristics of Natural Oil Seeps and Their Underwater Sources Based on Remote Sensing Data

How do microbial communities deal with chronic hydrocarbon presence in oil seep soils? Data from historical hand‐dug oil wells

AbstractHand‐dug oil wells, located in natural crude oil seep sites, are remnants of historical exploitation activities. Hydrocarbon pollution is regarded as the threat to soil ecosystem. On the other hand, there is no common environmental policies regarding these soils. The hypothesis was that natural attenuation processes might occur in seep soils since a diversified and stable bacterial community structure should be a result of its long‐term (thousands of years) adaptation to hydrocarbon exposure and should be associated with eventual utilization of these compounds. To obtained this goal, we compared the structure, composition, and hydrocarbon‐degrading potential of bacterial communities inhabiting soils with different hydrocarbon contents (seep, hydrocarbon‐impacted, pristine soils), which were collected in two habitats (forest, meadow). 16S rRNA sequencing and isolation of hydrocarbon degraders were performed. The contaminant's presence shaped distinct and unique community structure and composition, and it enhanced physiologically and functionally adapted microorganisms. The most abundant community members were bacteria revealing a strong contribution in genetic potential toward aerobic hydrocarbon transformation (i.e., Mycobacterium/Mycolicibacterium and Pseudomonas). The strong hydrocarbon degraders population suggests that natural biodegradation occurs in situ in seeps and mitigates the pollution impact on adjacent soils. Seep and hydrocarbon‐impacted soils are a great source for remedial bacterial populations. Twenty‐four genera of degraders were isolated; however, strains belonging to the Mycobacterium/Mycolicibacterium, Rhodococcus, and Pseudomonas taxa were common. Our results underline the need to include undervalued microbiological aspects in remediation projects' guidelines for chronically polluted environments. The knowledge regarding seep communities should help to evaluate more efficient remediation strategies for anthropogenic spills.

Mathematical Model of Shale Oil Seepage at Micro-Nano Pore Scale

Due to the unclear seepage mechanism for shale oil, it is hard to establish a reasonable mathematical model to describe the flowing law. In this study, a real shale pore structure was reconstructed with CT scanning and a numerical model of fluid flowing in the real shale pore is developed with COMSOL, based on the boundary slip at the micro-nano pore scale to study the causes of nonlinear seepage. A two-fluid model is employed to establish a simple mathematical model to describe the relationship between the flow rate and the pressure gradient using the effective slip length to account for the influence of pore structure and true slip, and the established model is verified with experimental data coming from Jiyang shale oil. The results indicated that the pore structure greatly affected the shape of the seepage curve. With the increase in displacement pressure, the percentage of fluid involved in the flow increased until it became stable, showing a changing trend from nonlinearity to linearity. The established model can fit the experimental data well and reasonably characterize the nonlinear seepage of shale oil.

Basin-scale estimates of thermal stress and expelled petroleum from Mesozoic-Cenozoic potential source rocks, southern Gulf of Mexico

The Campeche and Yucatan salt basins remain two of the least explored areas of the Gulf of Mexico basin. This study uses a grid of 23,600 line-km pre-stack depth migrated (PSDM) 2D seismic reflection profiles, shipborne gravity data, and open-source geologic information to model the thermal stress of four potential source intervals (Oxfordian, Tithonian-centered, Cenomanian-Turonian, lower Miocene). We performed map-based, and 1D thermal modeling along two margin-perpendicular transects, each consisting of five pseudo-wells tied to the regional grid of seismic reflection data. Our modeling takes into account thermal stress variations related to the depth of base lithosphere, crustal type and thickness, paleo-water depth, Jurassic salt thickness, and the transient heat flow effects related to recent clastic sedimentation. We predict that deeply-buried, salt-related minibasins along the outer marginal trough are mature for petroleum expulsion with deeply-buried Mesozoic source rocks within the oil window from late Paleogene to early Neogene time. The ‘lag time’ required for vertical oil migration explains why oil maturation occurred during the late Paleogene to early Neogene but active oil seeps are observed today at the sea surface. We predict that oil is present in subsurface traps in the deepest part of the outer marginal trough and we calculate that the Oxfordian source interval has expelled a cumulative 20 million bbl of oil equivalent [BOE]/km2 and that the Tithonian-centered source interval has expelled 67 million bbl of oil equivalent [BOE]/km2.

The Upper Albian and Cenomanian–Turonian source rocks, and related oil seeps around Extrusive Triassic salt structures, Northern Tunisia: geochemical characterization and implication for petroleum system assessment

The Upper Albian and Cenomanian–Turonian source rocks, and related oil seeps around Extrusive Triassic salt structures, Northern Tunisia: geochemical characterization and implication for petroleum system assessment

Oil slicks in the Gulf of Guinea – 10 years of Envisat Advanced Synthetic Aperture Radar observations

Abstract. The Gulf of Guinea is a very active area with respect to maritime traffic as well as oil and gas exploitation. Due to the failure of some actors to comply with environmental standards, this region has been subject to a large number of oil pollution episodes. This anthropogenic oil pollution is in addition to natural oil seepage from the ocean floor. This study aims to detect oil slicks in the Gulf of Guinea and to analyse their spatial distribution using synthetic-aperture radar (SAR) images. Previous works have already locally mapped oil slicks in this area; however, to our knowledge, this study is the first to achieve a global statistical analysis based on 10 years of radar images covering 17 exclusive economic zones (EEZs) in the Gulf of Guinea. The present study is based on a database of 3644 SAR images collected between 2002 and 2012 by the Advanced Synthetic Aperture Radar (ASAR) sensor onboard the European Spatial Agency (ESA) Envisat mission, and these images allowed the identification of 18 063 oil slicks. The spatial distribution of these oil slicks is available from Zenodo: https://doi.org/10.5281/zenodo.6470470 (Najoui, 2022b). The oil slicks detected in this work encompass both oil spills of anthropogenic origin and oil seeps of natural origin (natural oil reservoir leaks). A set of 100 georeferenced oil spills is also available from Zenodo: https://doi.org/10.5281/zenodo.6907743 (Najoui, 2022a).

Seismic Survey in Lesser Himalayan Thrust Belt, Western Nepal

Two hundred km of 2D seismic survey was carried out at the Lesser Himalayan Thrust Belts in Dailekh district, western Nepal. The main motivation is to elucidate the geologic relationship between the known oil and gas seeps, subsurface structure, and stratigraphy in the area. This is a challenging task which is from its extreme structural and geological complexity such as thrust faulting, tight folding, steep dip layers, and strong lateral variations in seismic velocity. Seismic data were acquired with SERCEL 428XL system and processed by GEOEAST computer software. In order to increase signal-to-noise ratio (SNR), suppress interference, and search for optimum acquisition parameters, a series of comparative tests on the different charge depth and size, group interval, CDP fold, geophone array, and single high-sensitivity geophone were conducted. We also tested 2S3L (two lines shooting and three lines receiving) wide line profiling. The results indicate that single hole with charge depth of 12 m, 4-16 kg charge size (less charge size for the densely populated areas), single high-sensitivity geophone, and 1S2L wide line profiling with 132 folds are the optimum acquisition parameters. On the basis of comparative process experiment, data processing workflow consisting of data preparation, prestack denoising, amplitude compensation, deconvolution, tomography static correction, velocity analysis, residual static correction, CRS stack, poststack migration, prestack time migration (PSTM), and prestack depth migration (PSDM) was selected. Maybe affected by problem of conflicting dip in complex media, CRS stack section does not show satisfactory geological characteristics. PSTM profile has moderate signal-to-noise (S/N) ratio; the shallow, medium, and deep continuous reflections can be observed in section. More details of the geological structures can be observed in PSDM section, especially in medium and shallow layers (less than 3000 ms or 4000 m), but PSDM method is more expensive and highly time consuming than that of CRS stack and PSTM. So, the PSTM section can be reasonably used for geological interpretation. By reference to field mapping, thrust characteristics, and MT data, the final interpretation to the PSTM section identified the interfaces of 6 geological units (Paleoproterozoic Nabhisthan Fm., Paleoproterozoic Dubidanda Fm., Neogene to Late Cretaceous Surkhet group, Late Carboneferous to Early Cretaeous Gondwana group, Mesoproterozoic Upper Lakharpata group, and Lower Lakharpata group) and delineated Main Boundary Thrust (MBT), Ramgarh Thrust (RMT), Padukasthan Thrust (PT), and Dailekh Thrust (DT). The bottom of Surkhet group which is our top target zone is about 4250 meters deep.

Discovery of a new Proterozoic paleo-reservoir and its oil source in the Jianchang Sag, Yanliao Faulted-Depression Zone, North China

The discovery of many Proterozoic primary oil and gas reservoirs around the world testifies that the Meso-Neoproterozoic petroleum is an energy resources realm worthy of attention. This paper firstly reports the occurrence of hydrocarbon accumulations in the Neoproterozoic Luotuoling Formation in the Jianchang Sag, the Yanliao Faulted-Depression Zone. Petrography observation shows that the lower sandstone member of the Luotuoling Formation contains yellow fluorescent liquid hydrocarbons, while the upper sandstone member is rich in solid bitumen. Further analysis of the biomarkers and isotopic compositions reveals that the upper and lower reservoirs have different oil sources. The bitumen in the lower reservoir is rich in C 19 –C 20 tricyclic terpanes (TTs), 13 α (n-alkyl)-tricyclic terpanes and rearranged hopanes, but lacks gammacerane and steranes and depletes 13 C isotope (−33.4‰ to −30.6‰). This is consistent with the properties of the Hongshuizhuang Formation source rocks. The upper reservoir bitumen has C 23 TT as the major peak among C 19 –C 23 TTs, lacks 13 α (n-alkyl)-tricyclic terpanes, has a low abundance of rearranged hopanes, obvious distribution of gammacerane and steranes, and depletion of the 13 C isotope; essentially the same as the Gaoyuzhuang Formation source rocks. The discovery of oil seepage in the Luotuoling Formation of the Han-1 well is a promising indicator of the Precambrian oil and gas exploration potential of the Jianchang Sag.

GEOCHEMICAL ANALYSES OF SEEP OILS IN THE SOUTHERN GAFSA BASIN, SW TUNISIA: REASSESSMENT OF REGIONAL HYDROCARBON POTENTIAL

The recent discovery of surface oil seeps in the Tamerza area in the west‐central Gafsa Basin (southern Tunisia) has prompted a re‐evaluation of the hydrocarbon potential of the region. In this paper, we report the results of analyses of seep oils by Rock‐Eval pyrolysis (n = 6) and gas chromatography – mass spectrometry (n = 4). The goals of the study were to assess the composition of the seep oils, to investigate the relationship between the seep oils and potential source rocks, and to highlight the significance of the seep oils for oil exploration in the region.In the Tamerza area, surface oil seeps have been recorded in numerous formations ranging between the Upper Cretaceous Abiod Limestone Formation and the lower Miocene Sehib Siltstone Formation. The results of this study showed that all the seep oil samples analysed in general had a similar geochemical fingerprint: for example, Pr/Ph values are lower than 1; a plot of Pr/n‐C17 (0.27‐ 0.36) versus Ph/n‐C18 (0.3‐0.8) indicates a marine source rock deposited under reducing anoxic conditions; and Ts/(Ts+Tm) ratios indicate that the source rock was thermally mature. Correlation studies suggest that the oils originated from Cenomanian‐Turonian shales corresponding to the informally‐named Bahloul equivalent formation. Oil expulsion from this source rock at the seep locations is inferred to have ended by middle Miocene time. However the main phase of folding occurred here in the Pliocene – early Quaternary, and the resulting anticlinal folds are not therefore prospective structural traps for hydrocarbons because they developed after migration had already ceased. Stratigraphic traps and salt structures in the region may be of greater exploration interest.A surface oil seep sample was also recovered from the Quaternary upper Segui Formation at Jebel Orbata in the east of the Gafsa Basin. Analysis of this oil sample showed that it has similar geochemical characteristics to the seep oils from the Tamerza area, but that it appears to have had a much more recent migration history. In the eastern Gafsa Basin, Pliocene – early Quaternary anticlinal structures could therefore constitute effective structural traps charged by the same Bahloul equivalent formation source rock.

Oil-gas two-phase seepage model in fractured carbonate reservoirs

Fractures are developed in fractured carbonate reservoirs. Traditional fracture characterization methods and seepage mathematical models cannot accurately describe the complex spatial distribution of large-scale fractures and their stress sensitivity, and do not consider the two-phase seepage of oil and gas, resulting in the dynamic analysis results have large errors. Aiming at this problem, a two-phase seepage model of oil and gas in fractured carbonate reservoirs considering stress sensitivity is established in this paper, and the semi-analytical solution of the model is obtained by using three-dimensional source function theory and finite difference method. The accuracy of the model is verified by comparison with commercial numerical simulation software, the production performance curve of oil and gas two-phase is drawn, and the influence of key seepage parameters of reservoirs and fractures on production performance is analyzed. The results show that the hybrid solution method of 3D source function and finite difference can realize the accurate and efficient solution of 3D discrete fracture oil and gas two-phase seepage model. The stress sensitivity effect leads to serious loss of fracture permeability and increase of fluid seepage resistance, which seriously affects the production of oil wells. The greater the fracture permeability, the higher the initial value of production, and the higher the position of the production curve. The longer fracture length, the higher the production curve in the early and middle production stages, and the slower the production decline.

Discovery of a new source-rock interval within the Pantokrator Formation, Ionian Zone, western Greece: Insights from sulfur speciation and kinetics analyses

The Ionian Zone in western Greece constitutes a Mesozoic sedimentary basin with petroleum potential. Numerous rock intervals with source, reservoir, and seal properties, along with several surface oil seeps have been identified. These rocks are linked to the northern portion of the Ionian Zone in nearby Albania, where similar rocks host oil and gas fields. This study describes a newly discovered source rock interval, the “Pantokrator Shales”, which lies within the Pantokrator Carbonates Fm, between the Rhaetian platform dolomite (“Pantokrator Dolomites” Fm.) and the lower Liassic platform limestone (“Pantokrator Limestones” Fm.) sequence. This interval is an important source rock for the petroleum system of the Ionian Zone in western Greece and Albania. The samples were acquired from outcrops and subsequently examined by programmed pyrolysis and organic petrography, as well as for sulfur speciation. The Pantokrator Shales Formation is comprised of ∼30 m of organic-rich shales alternating with less organic-rich light gray and thin-grained dolomitic sandstones and dolomite beds. The results of organic geochemical and organic petrographic analyses suggest high amounts (up to 38.68 wt% TOC) of Type I and mixed II/II-S organic matter with high generation potential (S1+S2 is up to 269.79 mg HC/g and HI of up to 790 mg HC/g TOC). Selected intervals analyzed have large amounts of labile organic sulfur. Pyrolysis Organic Sulfur (POS) ranges between 1.22 and 4.87 wt% on a whole-rock basis, with Sulfur Index (SI) values ranging from 116 to 174 (minimum SI for Types IS and IIS = 100), suggesting that the kerogen has already entered the oil window and generated liquid hydrocarbons in accordance with reflectance values measured on vitrinite (%VR: 0.60–0.65) and/or solid migrabitumens. Widespread conditions that favored the deposition and preservation of high amounts of oil-prone organic matter (including vulcanization processes of organic matter) existed locally in the basin during the Late Triassic to Early Liassic. This study showed that the stratigraphic position, the lateral and vertical extent, the organic matter quantity and quality, as well as the sulfur (S) content of the Pantokrator Shales, are comparable to intervals in nearby Albania, Sicily and the Italian peninsula. The high clastic input that was recorded may be associated with a regional shift in climatic conditions towards more seasonably controlled wetter conditions, as inferred from adjacent areas.

Organic Geochemistry and Oil-Source Correlation of Oil Seepages and Potential Source Rocks in Qiangtang Basin, China.

Different viewpoints on the oil-source correlation of the Angdarco paleo-reservoir in the Qiangtang Basin, northern Tibet, China, have seriously affected oil and gas exploration in this area. In this work, based on the results of biomarker analysis on six oil seepage samples and 11 potential source rock samples, the oil source of the oil seepages in the Angdarco area was examined by correlating the composition characteristics of n-alkanes, multiple factors of biomarkers, correlograms of biomarker parameters, and δ13C values of individual n-alkanes between oil seepages and potential source rocks. In addition, the sedimentary environment and thermal maturity characteristics of the potential source rocks were also discussed. The biomarkers indicated that the potential source rocks of the Lower Jurassic Quse Formation (J1q) presented characteristics of littoral sediments, deposited under suboxic conditions, while the potential source rocks of the Middle Jurassic Buqu Formation (J2b) were deposited in a marine sedimentary environment and presented characteristics of a low mature to mature stage. The potential source rocks of the Middle Jurassic Xiali Formation (J2x) were deposited in a low-salinity environment and presented characteristics of low-mature source rocks. Finally, the oil seepages of the Buqu Formation were deposited in a suboxic littoral environment and presented characteristics of a low mature to mature stage. It is concluded that the oil seepages in the Angdarco paleo-reservoir were mainly sourced from the Lower-Jurassic Quse oil shales. This work provides a scientific basis for the exploration of paleo-reservoirs in the southern Qiangtang Basin.

Comparison of the Ediacaran and Cambrian petroleum systems in the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China

Several different types of petroleum occurrences (including solid bitumens, oil sands, and oil seeps) are observed in various strata (e.g., the Ediacaran, Cambrian, Devonian, Permian, Triassic, and Jurassic strata) of the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China. However, the sources of these petroleum occurrences are still in dispute, because of the severe biodegradation, strong thermal alteration, and multiple tectonic movements. At present, such uncertainty in the potential source rock systems has hindered oil and gas exploration in the study areas. In this study, methods including catalytic hydropyrolysis and trace elements analysis were introduced into the oil-source correlation study, to avoid the influence of biodegradation and/or thermal alteration. The results show that the petroleum shows that occur in the Cambrian-Jurassic strata of the Tianjingshan area are characterized by low maturity (equivalent vitrinite reflectance (EqVRo) < 1.0%), light organic carbon isotopic ratios (δ13Corg values ranging from −34.0‰ to −38.4‰), a predominance of C29-steranes, enrichment in V and Mo, and strong depletion in Bi elements. In contrast, the solid bitumens in the Ediacaran Dengying Formation in the Micangshan area have high maturity (EqVRo > 2.0%), slightly heavier δ13Corg values ranging from −27.5‰ to −36.0‰, a predominance of C27-steranes, weak enrichment in Mo and strong depletion in Rb and Bi elements. In addition, the Ediacaran and the Lower Cambrian source rocks can be discriminated by the bound sterane distributions, trace element concentrations, and bulk δ13Corg values. Further oil-source correlation indicates that the petroleum shows in the Cambrian-Jurassic strata in the Tianjingshan area were dominantly derived from the Lower Cambrian Qiongzhusi Formation, while those in the Ediacaran Dengying Formation in the Micangshan area were mainly sourced from the Ediacaran Doushantuo Formation. Finally, the results of oil-source correlation, combined with the tectonic evolution history of the investigated areas further elucidate the occurrence and distribution of petroleum, which can assist in oil and gas exploration in the investigated uplifts and the adjacent areas.