Diagenesis and controlling factors of Oligocene Huagang Formation tight sandstone reservoir in the south of Xihu sag, the East China Sea Shelf Basin

Abstract

Considerable amounts of efforts have been made in search of depositional sand quality and diagenetic processes on the quality of low-permeability and tight reservoirs mainly due to the realization that these multiple factors exert profound controls on reservoir quality. This study evaluated the role played by original components and diagenetic processes on reservoir quality evolution of Oligocene Huagang Formation reservoir in the south of Xihu sag from braided river delta (H3, H4 and H5) to meandering river delta (H6 and H7). By a multi-disciplinary approach of thin section identification, scanning electron microscopy, grain size analysis, mineral phase analysis, and X-ray diffraction, the petrological characteristics, grain structure, and diagenesis of five of the layers (H3, H4, H5, H6, and H7) of Huagang Formation in the Western slope belt (WSB), the Western sag (WS) and the Central uplift belt (CUB) were studied. It was determined that the reservoirs are mainly composed of fine-medium sandstone (94.1%) and feldspathic lithic quartz sandstone (Q65.6F15.8L18.6). The E3H reservoirs in WSB and CUB Ⅲ structural belt are of conventional types with an average porosity of 13.2% and permeability of 30 × 103 μm2, and reservoirs in WS, CUB Ⅰ and CUB Ⅱ are mainly tight sand with an average porosity of 7.8% and permeability of 2.9 × 103 μm2. Furthermore, quantified characterization of diagenetic factors on porosity was revealed by multiple regression analysis to find out the depositional and diagenetic controls on reservoir quality. The effect of sorting, grain size, rigid lithic fragment, argillaceous content, compaction and feldspar dissolution have been identified as main contributing factors for reservoir quality evolution of E3h sandstones, notably post-depositional processes on the deep (3500–4500 m) and ultra-deep (>4500 m) tight sandstone reservoirs (DUDTSR) with high degree of diagenetic stage. Therefore, a better understanding of diagenesis will provide an efficient approach for evaluation and prediction of the DUDTSR.