Dynamic evolution characteristics of the “source-reservoir” integration of gray marl and its geological significance to unconventional gas: Insights from pyrolysis experiments

The marl–limestone rhythmic strata of the Permian Maokou Formation have been identified as hosts of unconventional gas reservoirs with “source–reservoir” integration. The lack of research on the pore structure evolution of organic-rich carbonate rock restricts gas exploration of these strata. Here, pyrolysis experiments were performed on the Mao-1 carbonate to simulate hydrocarbon generation, expulsion and diagenesis in gray marl from low maturity to overmaturity. The pore structure of this marl is dominated by mesopores and macropores, and the proportion of macropores increases gradually with temperature. The macropores are mainly pores in the organic matter and shrinkage microcracks. Additionally, micropores and mesopores, dominated by clay mineral interlayer pores and pyrite intergranular pores, are developed in the high mature stage and subsequently compressed in the overmature stage. The main contributors to the specific surface area are micropores and mesopores, which are conducive to natural gas adsorption. After the same pyrolysis treatment, the available porosity of grey marl is higher than that of marine/lacustrine shales, and exhibits an obvious decrease in the low mature–mature stage. These suggest that the abundant residual oil generated blocked the organic and inorganic pores in the gray marl, providing a pivotal material foundation for the gas generation. Micropores and mesopores developed during the high mature stage ensure the gas accumulation and preservation. The above indicate the organic-rich carbonate at the high mature–overmature stage (Ro = 1.7%–2.5%) in the Sichuan Basin may be a favorable exploration horizon for unconventional oil and gas.

Structural characterization and mass spectrometry fragmentation signatures of macrocyclic alkanes isolated from a Sydney Basin torbanite,Australia

Individual hydrocarbons identified to be macrocyclic alkanes in a torbanite from the Sydney Basin(Australia)were successfully isolated from its extracts using preparative gas chromatography and analyzed by NMR.Saturated cyclic structures were confirmed by single peaks in the NMR~1H and~(13)C spectra indicating single forms of H and C atoms exist in these biomarker molecules.This is consistent with the methylene unit in a ring system assignment of the macrocyclic alkanes and accounts for a formula of(CH2)n.The unusual molecular structures of these compounds are consistent with those that were identified from previous GC retention index data and co-injection with a standard supports the previous research.The mass spectral fragmentation behaviors of representative cyclic alkanes were further investigated by comparing them with the mass spectra of isolated individual macrocyclic alkanes.The characteristic fragment ions in the macrocyclic alkanes of(M–28)+and base peaks of m/z 97,111,125,etc.,can be assigned as being generated by simple a-/i-cleavage and hydrogen rearrangement.These fragmentation pathways combined with retention indices should assist in differentiating these compounds from monounsaturated alkenes and alkylated monocyclics having similar mass spectral characteristics in other geological samples.

Distribution and isotopic signature of 2-alkyl-1,3,4-trimethylbenzenes in the Lower Paleozoic source rocks and oils of Tarim Basin:Implications for the oil-source correlation

Series of 2-alkyl-1,3,4-trimethylbenzenes(ATMBs)were detected in most of crude oils and source rocks collected from various strata and locations of the Tarim Basin.They appeared to have heavy carbon isotopic signatures(δ13C,up to~-16‰)compared to those hydrocarbons from oxygenic phototrophic organisms,indicating that the unequivocal source of green sulfur bacteria(GSB)and photic zone euxinia(PZE)existed in the original environment.Considering the high paleoproductivity,the PZE may have enhanced the preservation of organic matter,which triggered the formation of extremely organic-rich source rocks with TOC up to 29.8%for the Lower Cambrian Yuertus Formation(€1y).The coexistence of ATMBs and the diagnostic products from secondary alterations(e.g.,abundant 25-norhopanes,thiadiamondoids,and diamondoids)indicated a stronger ability of anti-second-alterations.Combined with the results of quantitatively de-convoluting mixed oil,the oil-source correlation based on ATMBs from a large number of Lower Paleozoic samples of the Tarim Basin suggested that the abundant deep crude oil resources co ntained a dominant contribution(>50%)from the€1y source rocks.Therefore,the ATMBs,as diagnostic biomarkers indicating unequivocal precursors under special habitat conditions,might provide important insights for the exploration of deep Lower Paleozoic crude oils in the Tarim Basin.

Quantitatively unmixing method for complex mixed oil based on its fractions carbon isotopes: A case from the Tarim Basin, NW China

Deep mixed oils with secondary alterations have been widely discovered in the Tarim Basin,but current methods based on biomarkers and isotopes to de-convolute mixed oil cannot calculate the exact mixing proportion of different end-member oils,which has seriously hindered further exploration of deep hydrocarbons in the study area.To solve this problem,we constructed a novel method based on the carbon isotope(δ13C)of the group components to de-convolute mixed liquid hydrocarbons under the material balance principle.The results showed that the mixed oil in the Tazhong Uplift was dominantly contributed at an average proportion of 68% by an oil end-member with heavier d13C that was believed to be generated from the Cambrian-Lower Ordovician source rocks,whereas the mixed oil in the Tabei Uplift was predominantly contributed at an average proportion of 61% by an oil end-member with lighter d13C that was believed to be generated from the Middle-Upper Ordovician source rocks.This indicates that,on the basis of the detailed description of the distribution of effective source rocks,the proposed method will be helpful in realizing differential exploration and further improving the efficiency of deep liquid hydrocarbon exploration in the Tarim Basin.In addition,compared to traditional δ13C methods for whole oil and individual n-alkanes in de-convoluted mixed oil,the proposed method has a wider range of applications,including for mixed oils with variations in color and density,indicating potential for promoting the exploration of deep complex mixed oils in the Tarim Basin and even around the world.