Accumulation conditions and favorable exploration orientation of unconventional natural gas in the marl source rock of the first member of the Middle Permian Maokou Formation, Sichuan Basin

doi:10.3969/j.issn.1672-7703.2021.06.006

Abstract

Abstract: The early study regarded that the carbonate of the first member of Maokou Formation (hereinafter referred to as Mao 1 member) is a set of high-quality source rock in Sichuan Basin, but ignored its potential as an unconventional reservoir similar to the shale. Recently,commercial gas flows have been achieved of Mao 1 member in several exploration wells, probably indicating a new exploration field. Five aspects of geological studies are conducted, including oil and gas bearing properties in wells and oil/gas distribution, gas source analysis and evaluation of source rock conditions, lithofacies types and distribution of favorable lithofacies zones, reservoir performance and its distribution, gas accumulation conditions and enrichment characteristics, by which the unconventional natural gas is considered to be enriched of marl source rock of Mao 1 member. It is continuously accumulated in a large area in structural highs, slope and syncline. The natural gas is mainly supplied by source rock of Mao 1 member itself, which is featured by high TOC and high degree of gas enrichment, indicating the characteristics of gas accumulation inside the source rock and in-situ gas enrichment. Four types of lithofacies are mainly developed in Mao 1 member, i.e., the augen limestone lithofacies, eyelid bearing augen limestone lithofacies, augen bearing eyelid limestone lithofacies and eyelid limestone lithofacies. The study of rock physical properties shows that the eyelid limestone is the most favorable facies with reservoir space of mainly organic nanopores and dolomite intercrystal micropores. The reservoir porosity and permeability are generally less than 10% and 1mD respectively, which is a typical tight reservoir of low porosity and low permeability. The reservoir sweet spots are continuously distributed in the a and c sub-members of Mao 1 member vertically, and widespread laterally in the whole basin. The reservoir has distinct characteristics of overpressure and good gas bearing property. Combined with burial depth and reservoir distribution, the favorable exploration zones are comprehensively evaluated, and three Class Ⅰ and four Class Ⅱ favorable areas selected. The Nanchuan-Hechuan area has the best exploration potential, where risk exploration well is suggested to be deployed and volume fracturing of horizontal well be implemented for large-scale gas production.

CLC Number:

TE122.2

Jiang Qingchun, Wang Zecheng, Su Wang, Huang Shipeng, Zeng Fuying, Feng Zhou, Bi He, Lu Weihua. Accumulation conditions and favorable exploration orientation of unconventional natural gas in the marl source rock of the first member of the Middle Permian Maokou Formation, Sichuan Basin[J]. China Petroleum Exploration, 2021, 26(6): 82-97.

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