Search

The shale oil of the Lucaogou Formation in the Jimusaer Sag of the Junggar Basin was divided into two sweet spots for exploration and development. Crude oil in the upper and lower sweet spots comes from the upper and lower source rocks. After years of exploration, it has been found that the crude oil in the lower sweet spot has worse physical properties than that of the upper sweet spot. In this study, through the physical and geochemical analysis of crude oil in the upper and lower sweet spots, combined with the organic petrological observation of the upper and lower source rocks, the cause of the poor physical properties of the crude oil in the lower sweet spot has been identified. n-Alkanes in the saturated hydrocarbons of crude oil in the upper and lower sweet were complete while odd-to-even predominance was evident, indicating that the poor physical properties of the crude oil are unrelated to biodegradation. In addition, the correlation between the biogenic parameters and the physical properties of crude oil was analyzed, finding that the difference in crude oil is mainly related to the composition of biogenic precursors of upper and lower source rocks. Combined with organic petrological observation, the lower source rock was found to be rich in telalginite (green algae), which is therefore the primary reason for the difference in physical properties. In comparing results from the characteristics of crude oil biomarkers from both the upper and lower sweet spots, crude oils in the upper sweet spot are similar to each other, indicating that the enrichment of crude oil has experienced a certain migration. In contrast, the differences in biomarkers between the crude oils of the lower sweet spot were relatively large and changed regularly with depth, suggesting the self-generated and self-stored characteristics of crude oil enrichment. At the same time, it was found that the crude oil in the lower sweet spot is also affected by the maturity of adjacent source rocks under the condition of a consistent parent material source. Overall, it was determined that the lower the maturity of source rocks, the poorer the physical property of the crude oil produced. [ABSTRACT FROM AUTHOR]

Abstract Source rocks are found in the Eocene Liushagang Formation (El 1: bottom , El 2 and El 3:top members), in the Weixi'nan sag, Beibuwan Basin. The source rocks are mainly mudrocks, except El 2 , which consists of organic-rich shales at the bottom and mudrocks in the middle-upper part. Total organic carbon (TOC) and Rock-Eval analyses of 146 samples indicate that El 2 rocks have the best hydrocarbon generating potential, especially for organic-rich shales that contain up to 10% TOC with HI > 800 mg/g. The vitrinite reflectance analysis suggests that El source rocks are marginally mature to mature. Geochemical analyses were conducted on 18 El rock extracts and 18 oil samples from the WZ12 field. El 2 and El 3 source rock have similar organic facies and are distinct from El 1. El 2 and El 3 have higher C 27 βα (20R + 20S) diasteranes/C 27 αββ (20R + 20S) steranes (C 27 DiaS/C 27 S), Gammacerane/Hopane (G/H) and (C 28 + C 29) tricyclic terpane/((C 28 + C 29) tricyclic terpane + Ts) (ETR), and low C 23 tricyclic terpane/hopane (C 23 /H) and C 24 tetracyclic terpanes/C 26 tricyclic terpane (C 24 /C 26). These biomarker distributions indicate that El 2 and El 3 rocks were deposited under more alkaline-saline environment and had less import of terrestrial organic matter relative to El 1. El 2 can be distinguished from El 3 as having higher 4-methylsteranes/C 29 ααα steranes (4MSI) and G/H. WZ12 oils all exhibit similar compositions and display biomarker characteristics that correlate with the El 2 and El 3 rock extracts. 1D thermal and burial history model shows that El 2 and El 3 began to generate a great deal of hydrocarbon at ∼11.5 Ma and 23 Ma, respectively. At this stage, traps formed and faults actively extended downwards into the El 2 and likely into El 3. Oils expelled from El 2 and El 3 migrated upward into shallow intervals along faults then laterally along the structural slopes and accumulated in areas with good fault sealing primarily in the Miocene age Nx and Nj strata. Meanwhile, under the effect of overpressure in the middle of organic-rich shales, oils from El 2 may have migrated downward into reservoirs in the upper part of El 3. Lithologic traps in El 1 reservoir may be charged with relatively immature oil derived from El 1. Highlights • Source rocks in El 2 , especially organic-rich shales in the bottom of El 2 are the most important source rocks. • Sedimentary environment of different intervals of El have been distinguished. • WZ12 oils generated from oil El 2 and El 3 source rocks based on biomarkers parameters of saturated hydrocarbon. • Faults and overpressure may be responsible for the distribution of WZ12 oils. [ABSTRACT FROM AUTHOR]