Reservoir controlling factors of the Paleogene Oligocene Huagang Formation in the north central part of the Xihu Depression, East China Sea Basin, China

Abstract

Huagang Formation is a typical offshore, deep, low-permeability, tight sandstone reservoir with a low level of exploration and development and unclear control factors in the North Central area of the Xihu Depression in the East China Sea Basin. In this paper, we begin with the analysis of basic data, such as cores, casting thin sections, cathode luminescence, scanning electron microscopy, physical property statistics, grain size analysis, mercury penetration and magnetic resonance imaging, combined with field logging data from conventional logging, imaging logging and three-dimensional seismic imagery. The reservoir's physical properties were comprehensively discussed in terms of provenance, deposition, diagenesis and fractures. The results showed that the reservoir physical properties of the Huagang Formation were mainly affected by compaction, and the porosity and permeability decreased with the depth of burial. When the depth exceeds 4000 m, the porosity is basically <10% and the permeability is nearly <1 mD. The compaction above 4000 m was mainly controlled by provenance and depositional microfacies. The reservoir physical properties in the northern provenance is better than that of the east provenance. A high-energy braided channel is the best physical depositional microfacies. The reservoir formed by event sedimentation, such as from storms and floods, due to poor sorting, often containing mud and so on, with poor physical properties. Carbonate cementation only had a great influence on the local reservoir. Illite, illite/smectite and pore-filling chlorite were important factors affecting physical property deterioration. Pore-lining chlorite can promote the retention of primary pores. Below 4000 m, favorable reservoirs were mainly controlled by dissolution, resulting in dissolution pores being the main reservoir space. Microfractures played an important role in the properties of low-permeability and tight reservoirs. Microfractures themselves were easily dissolved into dissolution pores while being an important seepage channel for low-permeability and tight reservoirs. The denser the reservoir is, the more important the microfracture seepage is.