108 / 2022-02-17 19:35:14

Sequence stratigraphy and sedimentary evolution characteristics of the oolitic shoal gas reservoirs in the Lower Triassic Feixianguan Formation, eastern Sichuan Basin, China

Oolitic shoal; sedimentary facies; sequence stratigraphy; gas reservoir; Feixianguan Formation; Sichuan Basin

The granular shoal reservoirs of the Feixianguan Formation are among the principal reservoir types in the Sichuan Basin and possess important exploration potential for hydrocarbons. For that reason, the investigation of sequence stratigraphic framework and sedimentary facies characteristics of granular shoal sediments are of great significance in the study area. Based on the sequence stratigraphic division and the observation and analysis of fine sedimentary facies in single wells through thin sections, cores, and field outcrops, as well as 3D seismic profiles the sedimentary facies depositional model and a prediction map of favorable reservoirs distribution areas were established. The Feixianguan Formation can be divided into two third-order sequences (SQ1 and SQ2) with three sequence boundaries in particular unconformity boundaries as well as lithofacies transition and lithological boundaries. These sequences were further divided into five fourth-order sequences (Sq1, Sq2, Sq3, Sq4, and Sq5) and Sq3 was subdivided into Sq3-1, Sq3-2, and Sq3-3 to distinguish the main shoal body periods. We characterized five kinds of facies association in the research area, including restricted platform, open platform, platform margin, slope, and basin. It is mentioned that the lithology is principally composed of limestone, oolite, intraclast, bioclastic grainstone, dolomite, mudstone, and finally gypsum. Based on thirty-one 3D seismic profiles, the shoal bodies of the Feixianguan Formation are mainly developed in the upper part of the second stage. It shows two strong in-phase axes (phase I and II) of the shoal which mainly appear in troughs or weak peaks. A stable distribution appears in the dolomitic tidal flat area behind the shoal and is principally under strong peaks and weak amplitude. Intragranular pores, intergranular dissolved pores, moldic pores, fractures, and vugs are the primary storage spaces of the granular shoal reservoirs. The main favorable gas reservoir section is developed at the top of the platform marginal shoal on the exposed shoal, a high porosity oolite dolomite.