Geochemistry of Oils in the Terek‐caspian Foredeep and Prikumsk Swell, Ne Greater Caucasus, Southern Russia

Crude oil reserves in tight Middle and Lower Jurassic reservoirs are of increasing exploration interest in the central Sichuan Basin, SW China. However, the origin of these “tight oils” is poorly understood. In this study, sixteen samples of light oils/condensates from tight Middle and Lower Jurassic reservoir rocks were analysed using gas chromatography (GC) and isotope ratio mass spectrometry to investigate the oils’ origin and to classify them into genetic families. The tight oils can be divided into two families. Family I oils occur in the Gongshanmiao oilfield where reservoir units comprise the Da'anzhai Member of the Lower Jurassic Ziliujing Formation, the Lower Jurassic Lianggaoshan Formation, and the First Member of the Middle Jurassic Shaximiao Formation. Family I oils are characterized by relatively low values of the methylcyclohexane (MCH) and cyclohexane (CH) indexes, low values of Mango's parameter K2 for light hydrocarbon composition, and relatively negative δ13C values ranging from ‐30.8‰ to ‐28.9‰. Family I oils are inferred to be self‐sourced by lacustrine shales in the Da'anzhai Member and the Lianggaoshan Formation in the study area, both of which are rich in sapropelic organic matter. These source rocks also charged reservoirs in the First Member of the Shaximiao Formation. By contrast, the newly discovered Family II oils, which occur at the Jinhua oilfield and the as‐yet undeveloped Qiulin and Bajiaochang structures, are reservoired in the Second Member of Shaximiao Formation. Family II oils have higher values of the MCH index, CH index and Mango's K2 parameter, and δ13C values varying from ‐27.5‰ to ‐25.4‰. These oils have similar light hydrocarbon compositions and δ13C values to oils derived from source rocks in the Upper Triassic Xujiahe Formation which contain dominantly humic organic matter. Family II oils are therefore inferred to be derived from the coaly mudstones in the Xujiahe Formation.The different compositions of the tight oils in the First and Second Members of the Shaximiao Formation appear to be controlled by the distribution and thickness of source rocks in the study area. Thus, the Gongshanmiao oilfield where Family I oils occur in the First Member is close to the depocentre of source rocks in the Da'anzhai Member and Lianggaoshan Formation. These source rocks are inferred to have charged the First Member reservoirs which may also be present in nearby oil‐ and gas‐bearing structures such as Nanchong and Yingshan. By contrast, Family II oils occur in tight reservoirs in the Second Member in areas with thick successions of Upper Triassic Xujiahe Formation mudstone source rocks, such as the Jinhua oilfield. In areas where both source rocks are present such as the Zhongtaishan and Lianchi oilfields, Shaximiao Formation reservoirs appear to contain both Family I and Family II oils.

Section: Statistical Groupingmentioning

confidence: 99%

Composition of Light Hydrocarbons in Jurassic Tight Oils in the Central Sichuan Basin, China: Origin and Source Rock Correlation

Wei³

et al. 2022

Exploration Plays in the Caucasus Region

Tari

Blackbourn

Boote³

et al. 2021

Exploration efforts around the Greater Caucasus region started towards the end of the 19th century and established a wide range of petroleum play types in various basin segments around the orogen. All these plays are associated with the flanks of the inverted thrust-fold belt and the adjacent foreland basin systems, but display significant variation among the basin segments depending on the tectonostratigraphic units involved and the degree of exploration maturity. Whereas the same main source rocks have generated most of the hydrocarbons in all the basins (namely organic-rich shales in the Oligocene -Lower Miocene Maykop Group and the Eocene Kuma Formation), it is primarily the trapping style, both proven and speculative, which is responsible for the broad spectrum of play types observed. Eleven play type diagrams across six main petroleum provinces of the Greater Caucasus region are presented in this paper and summarize the current exploration understanding of the existing discoveries and potential new play targets. These play cartoons offer a prospect-scale summary of both mature producing and underexplored basin segments in a coherent visual manner, and are therefore intended to promote future exploration efforts in the Caucasus region. The testing of new play types requires the proper risking of the two most critical elements in the region: hydrocarbon kitchen effectiveness, and post-charge trap modification. The de-risking of these factors will require properly designed, fit-for-purpose acquisition of modern geological and geophysical data sets.

Geochemistry and Carbon Isotope Characteristics of Associated Gases From Oilfields in the Nw Greater Caucasus, Russia

Oblasov

Goncharov

Derduga³

et al. 2022

The NW Greater Caucasus and surrounding areas have a long history of petroleum production dating back to the mid‐19th century. However the origin of the oil and gas is still a matter of debate. This paper focusses on a study area to the north of the western Greater Caucasus covering the West and East Kuban Basins and neighbouring structural highs where oil and natural gas occur in reservoir units of Neogene, Paleogene, Cretaceous and Jurassic ages. The study is based on the results of organic geochemical and stable carbon isotope analyses of 21 samples of associated gases from 16 oil fields. The gas samples' compositions were investigated together with the δ13C values of C1 to C5 alkanes. Twenty of the gas samples were of thermogenic origin and one was mixed (thermogenic and secondary microbial). δ13C values for CH4 for all the gas samples varied over a wide range from ‐62.3 to ‐30.0 ‰, indicating major variations in the thermal maturity of the respective source rocks. In a previous study, three genetic types of oil were identified in the study area and are referred to here as Types 1, 2 and 3. Gaseous hydrocarbons associated with Type 1 oils were generated by source rocks in the Middle Eocene Kuma Formation and the Oligocene (lower) part of the Maikop Group which are inferred to have been deposited in anoxic, deep‐marine conditions. C2–C5 alkanes in gases associated with Type 1 oils have the lowest (most negative) δ13C values of the samples studied, and the lowest iso‐butane: n‐butane and iso‐pentane: n‐pentane ratios. These gas samples were recovered from wells in oilfields located in the south and SE of the West Kuban Basin, in the Akhtyrskaya fault zone which extends along the northern flank of the Greater Caucasus foldbelt. Higher δ13C values of C2–C5 alkanes were found in gas samples associated with Type 2 oils which were recovered from wells in the central West Kuban Basin. The associated source rocks are interpreted to have been deposited in weakly oxidizing subaqueous conditions and to contain mainly marine organic matter but with an input of higher land plant material. The precise age and stratigraphic identity of the source rocks is unresolved but they may comprise the Upper Oligocene – Lower Miocene (middle and upper) part of the Maikop Group. In gases associated with Type 3 oils, δ13C values of ethane to pentane are similar to those of gases associated with Type 2 oils. These gases were recovered from wells in oilfields located on the Adygeya Swell which divides the southern East and West Kuban Basins and from the Armavir‐Nevinnomyssk Swell to the north. The precise source of these gases and oils is not known but the oils are probably older than Late Cretaceous on account of the absence of oleanane.