PES Linda Wang
An overview of the largest buried hill gas and condensate field in the Bohai Gulf Basin China The offshore Bozhong 19-6 (BZ 19-6) gas and condensate field, which has more than 4 Tcf of proven gas in-place (GIP), is the first large-scale gas discovery in the buried hill play in the Bohai Gulf Basin. The field is operated by one of the national oil companies, China Offshore Oil Corp (CNOOC), and is currently under trial development. Prior to drilling BZ 19-6, several buried hill gas fields have been discovered in the basin but are smaller in scale. The Qianmiqiao field, situated onshore, has more than 900 Bcf GIP while the rest of the discoveries situated offshore are all smaller in scale. CNOOC drilled Bozhong 19-6-1 in 2017, with the well intersecting over 100 m of gas pay zones in the Archean buried hill reservoir and over 240 m of gas pay zones in the Lower Paleogene Kongdian Formation. Following the discovery, an additional 16 successful appraisal wells were drilled on the 118 sq km structure. BZ 19-6 field is situated in the southeastern part of the Bozhong Depression, at a water depth of around 24 m. It is 145 km south-east of the Tanggu District in the Tianjin City; 130 km north-west of Longkou City in Shandong Province.
BURIED HILL EXPLORATION HISTORY Exploration work on the buried hill play began in the 1970s. The first large Lower Paleozoic - Proterozoic carbonate buried hill discovery was made in 1975, with the onshore Renqiu oil field which has over 2.9 Bbbl of oil in-place (OIP). It has led to an exploration campaign targeting the same carbonate buried hill reservoir in the offshore Bohai Basin, but only a few small fields were discovered during 1980 - 1999, such as Bozhong 28-1, Caofeidian 1-6 and Caofeidian 18-2, etc. Drilling targets then shifted to migmatitic granite reservoirs in the Archean, Proterozoic and Misozoic formations, resulted in the discovery of two large migmatitic granite buried hill oil/gas fields Jingzhou 25-1S in 2002 and Penglai 9-1 in 2010. Exploration work then moved to the Bozhong Depression and targeted
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the natural gas potential in the buried hill reservoirs. Two deep buried hill traps in the southwest portion of the Bozhong Depression were drilled in 2012 (Bozhong 21-2-1) and 2014 (Bozhong 22-1-2) (see Figure 1). Both wells penetrated thick gas pays of 93 to 102 m in the Lower Paleozoic Ordovician carbonate reservoirs. Reservoir top depths were around 4,350 to 4,860 m. Gas quality though, had high CO2 content of up to 49% (Shi, Wang, et al. 2019). The discoveries of the Bozhong 212/22-1 gas fields demonstrates the prospectivity of buried hills for future gas exploration in this area of Bohai offshore. With further assessment and reprocessed 3D seismic data in the Bozhong Depression, the Bozhong 19-6 structure was then identified to have significant gas potential. The structure is close to the source kitchen and there are several faults
which include two regional deep faults (Tan-Lu Fault and ZhangPeng Fault) in the area which may help form a good fractured network for the reservoir. The Lower Tertiary lacustrine mudstones would then provide a good seal for the fractured reservoirs. Bozhong 19-6-1 on the Bozhong 19-6 structure was drilled from December 2016 to April 2017. It targeted a deep Archean buried hill reservoir and had a TD of 4,180m. The well penetrated 106 m of gas pay in the Archean section. It also made a breakthrough in encountering over 240 m of gas pay in the Eocene Kongdian sandy conglomerates and over 20 m of oil pay in shallower reservoirs. The following appraisal wells, BZ 19-6-2 and BZ 19-6-3, achieved average testing rate of 6.4 MMcf/d
