Geological characteristics and key exploration technologies of continental shale oil sweet spots: A case study of Member 2 of Kongdian Formation in the Cangdong sag in the Huanghua depression, Bohai Bay Basin

Under the general trend of stepping-up oil and gas exploration and development in China, the unconventional oil and gas resources such as shale oil and gas have become an important alternative. Abundant continental shale oil resources are developed in Member 2 of Kongdian Formation (Ek2) of the Cangdong sag in the Huanghua depression of the Bohai Bay Basin which has complex structure, strong heterogeneity, and large buried depth, thus, the geological characteristic of shale oil accumulation and key exploration technologies are investigated on base of the system coring of 635.8 m, centimeter-level fine description and analysis of over ten thousand samples, therefore, the “four accurate” technologies have been developed, including the accurate selection of sweet spot areas, accurate drilling of sweet spot layers, accurate fracturing of sweet spot layers and accurate measure of fracture-rich sections. The results show that the continental shale strata in Member 2 of Kongdian Formation can be divided into three types: felsic shale, limy dolomitic shale and mixed shale. The Member 2 of Kongdian Formation is characterized by diverse mineral components, abundant laminae, good source rocks and medium thermal evolution degree, tight reservoir with rich micropore and microfractures, high oil saturation and brittle mineral content, indicating good prospect of shale oil exploration. Through sedimentological study, TOC and Ro evaluation, brittle mineral calculation, and seismic fusion inversion, a total of 126 km2 of Class I sweet spots in Member 2 of Kongdian Formation was delineated comprehensively, which guides horizontal well deployment (Well GD1701H and Well GD1702H). The drilling rate of sweet spot of these two horizontal wells is up to 96% by tracing thin layer through fine calibration, locating compartment through fine inversion, ensuring window entry through precise positioning, and tracking sweet spot through adjustment. The "one-excellent and five-highs” criterion is used to select perforation points to realize differential designs of fracturing intervals and clusters. Micro-seismic and potentiometry monitoring show that the artificial fractures formed by volumetric fracturing are 300–400 m long and 120 m high, and control a volume of about 0.07 km3. The maximum daily oil production of two horizontal wells is 48 t and 66 t respectively to realize the economic exploitation of shale oil reserves controlled by fractures. The shale oil exploration in Member 2 of Kongdian Formation shows that the continental shale oil has great potential of exploration and development, and the above technologies are critical and effective for shale oil efficient recovery.