Characteristics and significance of the solid bitumen in the second member of Dengying Formation of Sinian system in the north of central Sichuan Basin, China

Abstract

With ongoing advancements in natural gas exploration, the second member of the Sinian Dengying Formation (Deng 2 Member) has emerged as a crucial region for providing natural gas reserves in the north of the central Sichuan Basin. The Deng 2 Member has a significant volume of reservoir solid bitumen. Geochemical characteristics and development mechanism of the solid bitumen were determined through measurements and analyses of the Deng 2 Member samples collected from primary exploration wells by using optical microscopy, SEM, gas chromatograph-mass spectrometer, and laser Raman spectrometer. The results show that the Deng 2 Member has a generally high content of solid bitumen, ranging from 2.96% to 5.13% on average in single wells. The solid bitumen mainly occurs as the fillings of dissolved pores (caves) and fractures, followed by intergranular pores, in the shape of spots, balls, plates, and veins dominantly. Diasteranes content and laser Raman spectrograms indicate that the solid bitumen is in the high maturity stage. The bitumen reflectance calculated by laser Raman spectroscopy is distributed between 2.49% and 4.09%, indicating the major source of the thermal cracking of crude oil. Solid bitumen in the Deng 2 Member has different contents of 21α(H)–C₂₉ norhopane, C₃₅ hopane, and C₃₄ hopane, and Ts/Tm values from the Deng 4 Member in the Gaoshiti Moxi area. It is inferred that some solid bitumen is from the source rocks of the Lower Cambrian Maidiping Formation and the Sinian Doushantuo Formation. Two stages of bitumen were developed in the Deng 2 Member, indicating possible two stages of oil filling and thermochemical sulfate reduction (TSR) in the geologic history.