Critical Condition of the Depth Limit of Oil Accumulation of Carbonate Reservoirs and Its Exploration Significance in the Lower Ordovician of the Tazhong Area in the Tarim Basin.

Carbonate rocks typically constitute porous media, making the study of hydrocarbon accumulation in carbonate reservoirs an essential area of research. In the Tazhong area of the Tarim Basin, specifically within the Lower Ordovician stratum exceeding 7000 m, effective reservoirs and industrial liquid hydrocarbon accumulations persist. However, the existence of a depth limit of oil accumulation (DLOA) for oil accumulation in carbonate reservoirs remains unclear, posing a challenge for explorers. This study quantitatively characterizes the critical condition of DLOA in deep carbonate reservoirs from the perspective of hydrocarbon accumulation dynamics. Through comprehensive experimental analysis, statistical assessments, and numerical simulations, it also forecasts the potential for deep oil exploration. Based on the results of mercury injection experiments on 350 carbonate rock cores collected from 19 drilling wells in the deep Lower Ordovician, it was found that the reservoir is compact and exhibits significant heterogeneity. The driving force for oil accumulation is the capillary pressure difference between the surrounding rock and the reservoir. A greater capillary pressure difference indicates improved oil-bearing properties within the reservoir. When the capillary pressure difference between the reservoir and surrounding rock reaches zero, oil accumulation cannot occur, marking the critical condition of DLOA. The critical pore throat radius for DLOA in the deep Lower Ordovician carbonate rocks in the Tazhong area of the Tarim Basin is determined to be 0.01 µm, and the DLOA is estimated at 9000 m. This study confirms that the maximum depth for the embedded Lower Ordovician carbonate reservoir in the Tazhong area does not surpass this limit. Consequently, oil exploration in deep carbonate rocks within this stratum is both feasible and promising. The findings from this study hold significant importance in scientifically predicting favorable areas for oil exploitation in deep layers and offer valuable insights into understanding the oil flow in carbonate rocks.

Application of mathematical statistics to shale gas-bearing property evaluation and main controlling factor analysis.

Gas-bearing property evaluation and main controlling factor analysis have remained a concern in shale gas research. The application of principal component analysis, an important mathematical statistical method, in gas-bearing property evaluation and main controlling factor analysis of the Longmaxi shale in the Weirong area, Sichuan Basin, was examined. The Longmaxi shale exhibits high heterogeneity, manifested in the organic matter abundance, mineral composition, and pore structure. Seven geological factors, including the temperature, pressure, TOC content, clay content, brittle mineral content, pore volume, and specific surface area (SSA), were selected in principal component analysis. Four principal components with geological significance, such as mineral composition, formation condition, pore structure, and organic matter abundance, were extracted through principal component analysis, and further constituted a comprehensive factor. Shale gas-bearing properties were evaluated according to the score of the comprehensive factor. The Longmaxi shale could be identified as exhibiting good, medium, and poor gas-bearing properties based on the comprehensive factor scores of these samples. According to each geological factor's contribution to the comprehensive factor, combined with geological analysis, it could be considered that gas-bearing properties are primarily controlled by pore volume, SSA, and clay content, followed by TOC content, brittle mineral content, temperature and pressure.

Molecular Characterization of NSO Compounds and Paleoenvironment Implication for Saline Lacustrine Oil Sands by Positive-Ion Mass Spectrometry Coupled with Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry.

NSO compounds mainly exist in geological bodies in the form of nonhydrocarbons and asphaltenes with abundant geological and geochemical information. Combined with the gas chromatography mass spectrometry (GC-MS) technology, positive-ion electrospray ionization Fourier-transform ion cyclotron resonance MS (FT-ICR MS) was used to understand the composition and distribution characteristics of NSO compounds in the oil sands of the Dongpu Depression and to explore their paleoenvironmental significance. The results show that n-alkanes are characterized by an even carbon number and phytane dominance, suggesting a saline lacustrine environment. Certain abundance of nC37 and ß-carotane, high gammacerane content, and low diasterane content are detected in the analyzed samples, reflecting the saline-reducing paleoenvironment for the organic matter. Nine types of heteroatom compounds are detected: N1, N1O1, N1S1, O1, O1S1, O2, O2S1, S1, and S2. The main compounds are S1 and N1 compounds, followed by O1S1 compounds. The double-bond equivalent (DBE) value of S1 compounds is mainly distributed between 3 and 12, and the carbon number is mainly distributed between 18 and 35. The DBE value of N1 compounds is mainly distributed between 4 and 14, and the carbon number is mainly distributed in the range 15-35. Among the S1 compounds, DBE3 compounds (thiophenes) have relatively more sulfur-containing carotenoids (C40). The abundance of C40 S1 and the ratio of pyridine and its homologue DBE4-8/DBE9-12 N1 compounds show a good contrast with the paleoenvironment indicators of gammacerane/C30 hopane and diasterane/regular sterane. They can be used as a reference for the paleoenvironment index. Maturity is another factor affecting the distribution of NSO heteroatoms in the oil sands. NSO compounds are enriched in the DBE area with higher condensation, and the main peak carbon shifts forward. As the maturity increases, the relative abundance of N1 compounds increases, the aromatization intensifies, and carbon is broken into short chains. The research results shed light on the potential application of NSO compounds in petroleum exploration based on FT-ICR MS.