DIAGENESIS AND RESERVOIR QUALITY EVOLUTION OF SHELF-MARGIN SANDSTONES IN PEARL RIVER MOUTH BASIN, SOUTH CHINA SEA

Abstract

A study of the diagenetic evolution of sandstones from Panyu low-uplift in the Pearl River Mouth Basin was carried out to unravel the controls on shelf margin sandstone reservoir quality. The reservoir rocks, Oligocene volcanic clastic sandstones of the Zhuhai Formation, have a burial depth of 2765 to 3440 m. 70 samples were studied using the granulometric analyses, X-ray diffraction (XRD) analyses, porosity and permeability measurements, SEM observations, and mercury porosimetry measurements. The sandstones are fine- to medium-grained lithic sub-arkose, sub-litharenite, and sub-arkose with an average framework composition of Q72F13L15. High content and strongly altered volcanic rock fragments are the most important detrital components. In this work, typical diagenetic processes such as compaction, VRF (volcanic rock fragments) and feldspar dissolution, carbonate cements, quartz overgrowth, and clay cements are observed. Cements in eodiagenesis stage mainly include clay coating, early calcite and siderite. The main mesogenetic cements include kaolinite, ankerite, and minor quartz. The dissolution of VRF, feldspar, and carbonate cements is the most distinguishing feature which had controlled porosity and permeability. Carbon and oxygen isotopes were measured to discuss the carbon sources for precipitation and diagenetic temperatures of carbonate cements extensively developed in sampled intervals. Pore types in the analyzed samples change from a mix of primary to secondary pores. Primary pores have been destroyed by mechanical compaction or occluded by quartz, clay, and carbonate cements. Secondary pores were generated by the dissolution of VRF and feldspars during burial history. Its volume varied from trace to 8% and greatly improved the porosity of sandstones with increasing burial depth. Porosity varies from 4% to 20% in this work and is the highest for the samples where carbonate cement contents are low and dissolution is well developed.