High-quality sandstone reservoir evaluation and prediction in a half-graben rift sag under the influence of igneous intrusions, Songliao Basin, Northeast China

Abstract

Tight sandstone reservoirs of continental lake basins are one of the most important types of unconventional hydrocarbon resources. The first member of the Yingcheng Formation (k1y1) in Baojia Sag, Songliao basin, is an important tight sandstone reservoir and also a region rich in hydrocarbon. However, volcanic rock intrusions increase the stratum thickness and change the characteristics of the reservoirs, thus making the distribution of high-quality reservoirs more difficult to predict. Therefore, according to the identification of sequence boundaries, the five 4th-order sequences of k1y1 in Baojia sag are divided, into SQ1–SQ5, which are mainly dominated by lacustrine and fan-delta depositional systems. The stratigraphic characteristics in the study area were affected by two stages of volcanic activity. In the first stage, a large area of tuff was developed in SQ5. In the second stage, sheets of intrusive rocks were formed in SQ1–SQ4. Through the identification of cast thin sections and Scanning Electron Microscopy (SEM) data, the types of reservoir micro-pores mainly develop primary pores, microfractures, intergranular, and intragranular dissolved pores. Given the complex strata under the interference of volcanic rocks, it is considered that sandstone with a 10–40% argillaceous content is a high-quality reservoir. The results showed that under the background of episodic tectonic activities, humid paleoclimate, and sufficient paleoprovenance, large-area fan-deltas of SQ2 and SQ4 are developed in both steep slope and gentle slope zones. Compared with SQ1 and SQ3, small-scale fan-deltas are formed under the background of a stable structure and arid paleoclimate. The reservoir quality gradually improved from SQ1 to SQ4, this is mainly due to the progressive reduction of reservoir argillaceous content, the gradual reduction of compaction, and the gradual enhancement of dissolution, resulting in the expansion of pore throats to improve pore connectivity. Geostatistical inversion based on argillaceous content was proven to be more effective than wave impedance inversion as it distinguishes lithology and predicts high-quality reservoirs disturbed by volcanic rocks. Comprehensive analyses show that the sandstone of SQ3 and SQ4 with a 10–40% argillaceous content is the highest quality of reservoirs, which interact with lacustrine source rocks and easily form “self-generated and self-stored” and “the lower-generation and upper-storage” patterns of hydrocarbon reservoirs. The prediction method and distribution law of high-quality reservoirs under the influence of volcanoes can provide guidance for hydrocarbon exploration and development.