Seismic Paleogeomorphology of a Platform Carbonate System and its Influence on Quality of Deeply-Buried Dolomitized Reservoir in Cambrian Longwangmiao Formation, Moxi-Gaoshiti Area, Sichuan Basin, China

Abstract

The giant Moxi-Gaoshiti gas field in Sichuan Basin is the largest single gas field discovered in China so far. It is produced from deep (4,500-5,000 m) dolomite reservoirs in Cambrian Longwangmiao Formation. Reservoir quality was inferred to be influenced by dolomitization of oolite grainstones deposited in a carbonate platform system. In this study, we integrated outcrop, core, wireline log and 3D seismic data in a seismic-sedimentologic workflow to investigate the paleogeomorphology of the carbonate depositional system and its influence on reservoir quality. Outcrop, core and wireline-log data revealed a variety of the depositional facies in the higher-order carbonate sequences, including evaporite lagoon, open platform, platform-margin oolite shoal, slope, and basinal facies. High-quality reservoir rocks formed in the third-order highstand system tract in the (Chuanzhong) platform. Development of dolomitized reservoirs was observed to closely relate to high-frequency sequence framework and to exposure of the platform during sea-level falls. A seismic paleogeomorphologic map was generated by flattening Longwangmiao seismic events in 3D seismic volume against a nearby reference seismic event in the Paleozoic. With minimal tectonic disturbance after the disposition, the map was inferred to represent relative accommodation at the time of deposition. Dipping to the south, a regional low-angle (0.2-0.5°) ramp appears to fit the basin architecture; platform geometry was observed on the inner to outer platform. Seismic stratal slices made along the reservoir interval demonstrated many elongate to sheetlike high-amplitude anomalies on or nearby the paleogeomorphologic highs, revealing the distribution of high-quality dolomitized grainstone or packstone reservoirs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016