Sweet spot evaluation and exploration practice of lacustrine shale oil of the second member of Kongdian Formation in Cangdong sag, Bohai Bay Basin

Abstract

Based on core, thin section, X-ray diffraction, rock pyrolysis, CT scanning, nuclear magnetic resonance and oil testing data, the macro and micro components, sedimentary structure characteristics, of Paleogene Kong 2 Member in Cangdong sag of Huanghua depression and evaluation standard and method of shale oil reservoir were studied to sort out the best shale sections for shale oil horizontal wells. According to the dominant rock type, rhythmic structure and logging curve characteristics, four types of shale lithofacies were identified, namely, thin-layered dolomitic shale, lamellar mixed shale, lamellar felsic shale, and bedded dolomitic shale, and the Kong 21 sub-member was divided into four quasi-sequences, PS1 to PS4. The PS1 shale has a porosity higher than 6%, clay content of less than 20%, and S1 of less than 4 mg/g; the PS2 shale has well-developed laminar structure, larger pore and throat size, better connectivity of pores and throats, high contents of TOC and movable hydrocarbon, S1 of over 4mg/g, clay content of less than 20%, and porosity of more than 4%; PS3 shale has S1 value higher than 6 mg/g and clay content of 20% - 30%, and porosity of less than 4%; and PS4 shale has lower TOC content and low oil content. Shale oil reservoir classification criterion based on five parameters, free hydrocarbon content S1, shale rhythmic structure, clay content, TOC and porosity, was established. The evaluation method of shale oil sweet spot by using the weighted five parameters, and the evaluation index EI were proposed. Through comprehensive analysis, it is concluded that PS2 is best in quality and thus the dual geological and engineering sweet spot of shale oil, PS3 and PS1 come next, the former is more geologic sweet spot, the latter more engineering sweet spot, and PS4 is the poorest. Several vertical and horizontal wells drilled in the PS2 and PS3 sweet spots obtained high oil production. Among them, Well 1701H has produced stably for 623 days, with cumulative production of over 10000 tons, showing bright exploration prospects of Kong 2 Member shale oil.