STABLE ISOTOPE GEOCHEMISTRY OF LACUSTRINE TIGHT OIL SYSTEM IN THE CENTRAL SICHUAN BASIN, CHINA: CORRELATION, FRACTIONATION, AND EVIDENCE FOR THE CONTRIBUTION OF WATER HYDROGEN INTO OIL GENERATION

The Daanzhai Member of the Lower Jurassic Zhiliujing Formation is a shallow to semi-deep lacustrine deposition system and a primary oil production horizon in Sichuan Basin. The mudstone and shale in the middle section of Daanzhai Mbr. characterized with high abundance of organic matter (TOC=0.7~2.8%), medium maturity (Ro=0.7~1.4%), and extensive spatial distribution (depth=60~120m, area=7×10⁴km²), forms good oil-prone source rock. However, the shelly limestones in upper and bottom sections, including clastic shell layers interbeded within shale, are very tight (porosity=0.13~3.18%, permeability=0.0001~0.3mD). The conventional development aiming at tight limestone reservoir has maintained continuous but low oil yield since 1950s.

At present, research, exploration and production test on Jurassic play in the central Sichuan Basin are turning to the way of tight (shale) oil. This study selected a continuous Daanzhai core of well Penglai 10 and oils from ambient Daanzhai horizons and focused on stable carbon and hydrogen isotopic analysis on kerogen, oil, extractable organic matter (EOM), SARA compositions, and n-alkanes.

Stable isotope characterization and correlation in this lacustrine shale-oil system draw the following conclusions. 1) Oils from upper, middle, and bottom of Daanzhai limestone layers, with lacustrine characters, have similar isotopic compositions on component and molecular levels. 2) TOC>1% is the abundance limit for effective source rock that is located in the middle section of Daanzhai Mbr. Together with parameters from Rock-Eval and indication of “crossover effect” (Jarvie, 2012), the middle section of Daanzhai Mbr. could be also the producible shale oil layer. 3) Carbon and hydrogen isotope ratios of saturate, aromatic and resin (NSO) components and n-alkanes of oils are consistent with these of EOM from effective source rocks, implying secondary migration does not cause isotopic fractionations and supporting nonexistence biodegradation, water washing and cracking in reservoir. 4) δ¹³C values of organic components from effective source rock follow the normal isotope type curve, while δD values vary in different order as asphaltene>resin>kerogen>aromatic>EOM>=saturate, suggesting that water-hydrogen was incorporated into hydrocarbon generation.