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Geochemical Characterization of Daanzhai Shales in Fuling Area, Eastern Sichuan Basin, China

Abstract

Fuling Area is located in Eastern Sichuan Basin. Lacustrine shale of Daanzhai Member of Mid-Jurassic Ziliujing Formation (J2zD) in Fuling Area serves as good source rock and produces both oil and gas. To figure out the geochemical characteristics and determine the petroleum potentieal of Daanzhai Shale, fourty-one (41) shale samples from two wells and six oil samples from six wells were characterized via multiple geochemical approaches. Vitrinite reflectance (Ro), total organic carbon (TOC) content, and rock-eval pyrolysis of shale samples were carried out. Saturate and aromatic fractions of shale extracts and oil samples were evaluated by Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). More than 70% samples have the TOC over 1 wt%. The kerogen contents and rock-eval parameters indicate the organic matters of Daanzhai shales were mainly Type II 1. Vitrinite reflectance data suggest that most samples are mature for hydrocarbon generation. Combined all the factors, Daanzhai Shales are classified to good source rocks. Vertically, the second bed of Daanzhai Member seems the best target with most favorable geochemistry characteristics. Compared through the whole research area, the northeast performs better than the southwest. The quantities of the group components of shale extracts and oil samples show that most of the samples are characterized by high content of alkanes. With the Ro of 1.1–1.3, it is odd that there are few biomarkers in alkanes, even after the n-alkanes were expelled. We tried a number of common and uncommon sterane and terpane hydrocarbons but they are not there. However, the aromatic parameters of maturity and organic type are regular and consistent with the former results.