Characteristics and Differences Analysis for Thermal Evolution of Wufeng–Longmaxi Shale, Southern Sichuan Basin, SW China

Abstract

The marine shale of the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation is the main source rock and the target of shale gas exploration in the southern Sichuan Basin. The maturity of organic matter (OM) is a vital indicator for source rock evaluation. Due to the lack of vitrinite, the organic matter maturity of the Wufeng–Longmaxi Formations in the southern Sichuan Basin is difficult to accurately evaluate. In total, 33 core samples of the Wufeng–Longmaxi Formations in the southern Sichuan Basin were selected to observe the optical characteristics of solid bitumen and graptolites and measure their random reflectance. Simultaneously, Raman spectroscopic parameters of kerogen were also used to quantitatively analyze the change in maturity. By using Raman spectroscopic parameters as mediators, conversion equations between graptolite random reflectance (GRor) and equivalent vitrinite reflectance (EqVRo) were established. Taking the calculation results of EqVRo as constraints, the tectono-thermal evolution history of Wufeng–Longmaxi Shale in the southern Sichuan Basin is constructed through basin modelling. The results show that the maturity of Wufeng–Longmaxi Shale in the western Changning, Luzhou-western Chongqing, eastern Changning and Weiyuan areas decreases successively. The EqVRo falls in the ranges of 3.61%~3.91%, 2.92%~3.57%, 3.08%~3.25%, 2.41%~3.12%, and the average EqVRo is 3.73%, 3.30%, 3.18% and 2.80%, respectively. Thermal evolution in western Changning was controlled by the thermal effect of the Emeishan mantle plume and paleo-burial depth, while the thermal evolution of other areas was mainly controlled by paleo-burial depth. This study provides a reliable parameter for the evaluation of thermal maturity and makes a more accurate calibration of the maturity of the Wufeng–Longmaxi Formations in the southern Sichuan Basin; it also expounds the factors for the differences in thermal evolution in different parts of the area.

Strata Uplift Controlled Deep Shale Gas Accumulation Modes: A Case Study from the Weiyuan Block, Sichuan Basin

“…The southern region of the Sichuan Basin has witnessed numerous uplifts, erosions, burials, and depositions. The erosion thickness of the Weiyuan Block was close to 800 m in Caledonian. , The Soochow movement lasted for a short time in the Weiyuan area. The uplift and erosion of this area were primarily regulated by the eruption of Emeishan basalt.…”

“…Besides, in the study area, the pressure coefficient of Longmaxi Formation was significantly related to the burial depth as a logarithmic function, and the correlation coefficient was 0.7533 (Figure ). Therefore, the reservoir pressure at varying burial depths was calculated using the correlation between pressure coefficient and burial depth, and the calculation of temperature can be performed by the geothermal gradient, which is 28 °C/km in the Weiyuan Block at present …”

“…The first stage is from the Late Middle Cretaceous to the end of the Paleogene (97–29 Ma). During this period, the strata uplift in the Sichuan Basin was relatively slow, and the erosion in the Weiyuan area was approximately 1000–2000 m. , The second stage is from the Neogene to the present. During this period, the strata were rapidly uplifted.…”

“…During this period, the strata were rapidly uplifted. The denudation in the Weiyuan Block was 1000–2500 m, with the highest uplift of the Weiyuan anticline. ,, …”

“…The reservoir pressure, pressure coefficient, and gas content used in this study were provided by the Shale Gas Research Institute, the Southwest Oil and Gas Field Branch of PetroChina. The pore size characteristics of the Longmaxi Formation shale were referenced from previously published academic papers. − In addition, the parameters used in the basin simulation were adopted from the study by LYU et al, Jiang et al., and Deng et al…”