Fluid system and pressure evolution study based on isotope and fluid inclusion geochemistry: a case study on the Sinian Dengying and the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structure, Central Sichuan Basin

Abstract

Systematic microscopic observation and analysis reveal multi-stage mineral fillings and fluid charging in the Sinian (Ediacaran) Dengying and the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structure, Central Sichuan Basin. Reservoir pore space is filled with five different stages of minerals from the edge to the center: fine crystalline dolomite, rimmed bitumen, coarse crystalline dolomite, stripped or oil droplet-like bitumen, and calcite or quartz, respectively. Sr, C, and O isotopic analysis for fine crystalline dolomite and the late stage calcite and quartz filled in reservoir pore space indicate that, prior to oil-generating window of the Cambrian Qiongzhusi Formation, the Sinian Dengying and the Cambrian Longwangmiao Formation were within the same connected and open fluid system. But after the gas-generating window, the Cambrian Longwangmiao Formation evolves gradually into a relatively independent and closed fluid system. By restoring the paleo fluid pressure, it is found that the paleo fluid pressure coefficient of the Sinian Dengying Formation has gone from 1.0, 1.1 to 1.2, 1.2, 1.2 to 1.3, and 1.0 to 1.1 at different evolution stages of source rock (at corresponding Ro value of 0.5, 1.3, 1.7, 2.0, and over 2.8, respectively), and the paleo fluid pressure coefficient of the Cambrian Longwangmiao Formation has varied from 1.0, 2.1 to 2.2, 1.8 to 1.9, 1.6 to 1.8, and 1.55 to 1.75. The pressure restoration result reveals that overpressure has never occurred in the Sinian Dengying Formation during the geological history, but overpressure commonly occurred in the Cambrian Longwangmiao Formation, and the values varied at different evolution stages of source rock. This pressure evolution process also demonstrates that the Sinian Dengying and the Cambrian Longwangmiao Formation, prior to the oil-generating window, were within an identical normal pressure fluid system. Meanwhile, it also matches well with the present exploration discoveries: the overpressured gas pools of the Cambrian Longwangmiao Formation are preserved up to now and hold hundreds of billions cubic meters of gas reserves, while the Sinian Dengying Formation contains residual normal pressure gas pools that formed after the adjustment of natural gas over a large area.