Sedimentology, diagenetic evolution, and sweet spot prediction of tight sandstone reservoirs: A case study of the third member of the Upper Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

Abstract

Substantial progress has been made in the exploration of the tight petroleum resources in Bohai Bay Basin, as a series of tight oil reservoirs were discovered. Among these reservoirs, a new breakthrough was achieved in the Nanpu Sag, proving a previously unconfirmed prospect of tight oil. Mineralogical, petrographic and fluid inclusions analyses were performed to investigate physical properties (composition, pore system, porosity, and permeability), controlling factors, and features of tight sandstones of the Upper Paleogene Es3 (the third member of the Shahejie Formation) in the Gaoshangpu Structural Belt of the Nanpu Sag. Our study indicates that the Es3 tight sandstones are mainly feldspathic litharenites and had low values for porosity (average value 10.1%) and permeability (average value 0.8 mD). The pore system is dominated by secondary dissolution pores (average thin porosity 3.1%). The deterioration of porosity is greater due to compaction (average compaction rate 68.8%) rather than cementation (average cementation rate 16.3%). Two peaks in the homogenization temperature were observed; the first ranges from 116 °C to 136 °C and exhibits yellow fluorescence, while the other peak varies from 156 °C to 165 °C, showing blue fluorescence, which indicates two stages of hydrocarbon charging at 26 Ma and 3 Ma, respectively. Tight sandstone is identified during the Mesodiagenesis phase. Eodiagenesis included compaction, precipitation of calcite cements and quartz overgrowth. Mesodiagenesis mainly exhibited compaction and grain dissolution (mainly feldspar). The main controlling factors can be summarized as follows, with the second factor being the most important: (a) a high energy deposition environment (distributary channels); (b) intense dissolution; and (c) early hydrocarbon charging and overpressure. This study provides a new prospect of the tight sandstone petroleum system in northeastern China.