Integrated Assessment of Thermal Maturity of Upper Ordovician–Lower Silurian Wufeng-Longmaxi Shale in Southern Sichuan Basin, China

Abstract

The marine shale located at Upper Ordovician(O₃w)-Lower Silurian Longmaxi(S₁l) formation is the most important source rock and shale gas exploration target in Sichuan basin, China. Reliable organic maturity is crucial for hydrocarbon generation evaluation, and also is one of key factors that play an important role in determining the viability of these shales as both source and reservoir. Uncertainties in thermal maturity evaluation would occur if we adopt equivalent vitrinite reflectance based on the conversion formulas of bitumen reflectance because of pre-Devonian shales absence of vitrinite macerals.

In this article, we adopted different test methods to integrate assessment of thermal maturity parameters from 8 drilling cores and 10 outcrop samples on the basis of accurately distinguishing macerals from their characteristic shapes and morphologies. Graptolites with clear morphology and pore-filling amorphous bitumens are abundant in Wufeng-Longmaxi shale. In order to reduce the uncertainty of maturity evaluation, a variety of maturity indicators are used to evaluate maturity and used to constrain maturity range. The methodological approach consists of measurement of organoclasts optical reflectance(mainly graptolite, bitumen and vitrinite-like maceral) and Raman spectral analysis of graptolite, bitumen and kerogen.

The results show that average reflectance of graptolite is between 2.49% and 4.05%, and the corresponding equivalent vitrinite reflectance range is between 2.33% and 3.78%, suggesting that thermal maturity with a wide range sit in over mature stage. Graptolite reflectance has a better regional distribution consistency than bitumen reflectance and gradually increases from southwest to northeast in southern Sichuan basin, whereas bitumen reflectance as a poor maturity parameter due to solid bitumen mobility and various origin. As a complement method, Raman spectroscopy allowed us to improve the definition of thermal maturity as a complement method especially when conventional maturity measurements is unclear. Raman band separation (RBS) between G peak positon and D peak position is the most effective maturity indicator. However laser wavelength-dependence of the Raman D peak position is the factor for observed differences in absolute RBS values as a function of thermal maturity among different laboratories and it is necessary to establish universal correlation and curve fitting procedures between RBS and thermal maturity for the future study.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018