Characteristics of the newly found oil-bearing sandstone in the Denglouku formation of the northern Songliao Basin, China

Abstract

Over the past 50 years, exploration in the northern Songliao Basin has revealed that the sandstone in the Denglouku Formation is a tight gas reservoir. Oil-bearing reservoirs with good physical properties have only recently been discovered and have become the new focus of petroleum exploration. We carried out thin section observation, X-ray diffraction analysis, particle size analysis, mercury intrusion test, and stratigraphic burial history based on 377 core samples collected from the Denglouku Formation in the Shuangcheng area, where oil exploration has made a significant breakthrough. The results revealed the reservoir rock to be feldspathic sandstone and lithic feldspar sandstone, with the dominating interstitial materials of carbonates and mud matrix. The grain size ranged from silty sand to coarse sand. The pore types were primary intergranular pores and partial secondary dissolution pores. The porosity was generally 5%–20%, and the permeability was in a wide range of 0.1 mD to 2000 mD. Tectonic uplift prevented the sandstone from being deeply buried, which is an important reason for the good quality of the reservoir. Hydrocarbon filling improved the reservoir quality not only by forming dissolved pores but also by inhibiting carbonate cementation. The mud matrix and the carbonate cement decreased the reservoir quality, causing heterogeneity of the reservoir. This study indicated that the displacement pressure was effective in the classification of reservoirs that were characterized by good physical property and strong heterogeneity. The Denglouku reservoirs were divided into three classes. Favorable reservoirs (Class I and Class II) showed low displacement pressure because of the low mud matrix and carbonate cement and well-developed primary pores.