8 minute read
IHS Markit
from PESGB January/February 2021
by GESGB
PES
Linda Wang
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 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
Figure 1 - BZ19-6 gas field location map (modified after Shi, Wang, et al, 2019)
Figure 2 - Cross section of the BZ19-6 gas field in Bohai Gulf Basin (Modified after CNOOC 2018)
of gas and 1,000 b/d of condensate. Three wells, BZ 19-6-1, BZ 19-6-3 and BZ 19-6-5, were drilled in the southern part of the field and intersected Kongdian reservoirs, with thicknesses of 404 m, 390 m and 622 m respectively (Shi, Wang, et al, 2019) (see Figure 2). A total of 11 appraisal wells had been drilled by the end of 2018 and confirmed more than 4 Tcf (120 Bcm) of proven gas in-place and over 600 MMbbl (100 MMcm) of proven condensate in-place in an area of 118 sq km. Average thickness of the gas pay zones reached approximately 230 m. Appraisal well Bozhong 19-6-4 achieved high test rates averaging about 11 MMcf/d and 1.9 Mbc/d of condensate. Appraisal well Bozhong 19-6-7 was the deepest well drilled in the offshore Bohai Basin with a TD of 5,508 m. Four appraisal wells completed in 2019 all penetrated over 100 m of gas pay which added more than 700 MMbbl of oil equivalent of proved inplace volume.
PES
1. GEOLOGICAL FEATURES STRUCTURE AND TRAPS:
The formation and evolution of the BZ 19-6 low-relief trap came from four tectonic stages, including the Indosinian to early Yanshanian compressional thrust orogenic stage, the mid-Yanshanian extensional faulting uplift stage, the late Yanshanian reverse folding uplift stage and the Himalayan reformation burial finalizing stage. The BZ 19-6 low-relief trap is divided into the north and south sections, with the top depth ranging from 2,825 to 5,475 m. The trap has a general closure of 251 sq km and height of between 75 to 1,225 m (Xu, Yu, et al, 2019) (see Figure 3).
HYDROCARBON PROPERTIES
Bozhong 19-6 gas has extra-high condensate content with gas-oil ratios (GOR) in the range of 5,300 to 9,300 scf/bbl. Condensate content is more than 700 g/m3. Methane content of the gas averages over 75%, with about 10% of CO2 content in average (Shi, Wang, et al, 2019).
RESERVOIR AND SEAL
The Archean reservoirs comprise of metamorphic rocks, granites, amphibolites and migmatites. The fractured reservoirs are weathered, with high-quality reservoirs concentrated within a 120 m interval from the top of the buried hill. The fractures are characterized by multiple phases: weathered leaching caves and caverns (fracture), intergranular fracture of mineral particles and structural fractures. Core analysis shows that the Archean reservoir has porosity ranging from 0.2% – 22% (average of 4.4%), permeability ranging from 0.003 – 615 mD (average of about 5 mD), indicating strong heterogeneity (Xu, Yu, et al, 2019). The reservoirs are sealed by the overlying mudstones of the Shahejie and Dongying formations, which are regional seals with thickness of up to 1,200 m (Xue, Li, 2018).
SOURCE ROCK AND ITS MATURITY
Bozhong 19-6 confirmed the source rocks in this area have generation potential of up to 67 Tcf (Xie, Yuhong, et al 2020).
A good mix of Type II dominated source rocks are present in organicrich lacustrine mudstones of the Shahejie 3, 1 and Dongying 3 formations, with total thicknesses of 500 m to 2,500 m. The source rocks are mature to over-mature with Ro of up to 2.5% (Shi, Wang, et al. 2019).
Among the source rocks, the lacustrine mudstones of the Shahejie 3 Member proved to be the most important source interval in the Bozhong sag. The source rocks are mixed with humus and sapropelic, deeply buried at 4,500 to 7,000 m in depth and has Ro of about 1.2% - 3.5%. The source rocks have high total organic carbon (TOC) contents that range from 0.52 – 8%, with potential hydrocarbon content S1+S2 of over 17 ppm in average (Shi, Wang, et al. 2019). Migration distance in the Bozhong Depression is thought to be fairly short due to rapid facies changes and complex fault development. Hydrocarbons, generated from the Shahejie Formation, migrated to the traps via basement-involved faults and unconformities.
2. DEVELOPMENT AND PRODUCTION
Bozhong 19-6 field is under a pilot development phase which commenced production in October 2020. Future development will be in three phases based on the pilot development results.
The first development well BZ 19-6A4H was drilled in early 2020 in the pilot development block. Seven production wells and one water injection well are proposed in the pilot development project, and these wells will be drilled to an average depth of 5,170 m. First gas was announced on 22 October 2020 and production is expected to peak at about 35 MMcf/d of gas and 5,720 b/d of condensate by the end of 2020. Under the pilot development project, the field will be developed with a wellhead platform BZ19-6WHPA, two subsea mixing transporting pipelines and one subsea cable, in addition to fully utilizing the existing infrastructure in the BZ 19-4, BZ 13-1 and BZ 25-1 fields. The gas produced from the field will be transported via existing pipeline infrastructure, to the onshore Boxi terminal in Tianjin City for processing, and qualified gas will be supplied to the local market through the existing pipelines. Condensate will be stored on the FSO that is located at the BZ 25-1 field and exported to the domestic market using shuttle tankers. Future development will involve drilling over 300 development wells in the field and additional platforms will be built to support the production. The third phase of development is anticipated to be completed by 2025. The platforms will consist of 20 wellhead platforms, three central platforms and three gas injection platforms. Gas will then be supplied to the Huabei Region which covers Beijing, Tianjin and Huabei Province.
3. CONCLUSIONS
With the large scale discovery made at BZ 19-6, gas potential in the Archean buried hill play in the Bohai Gulf Basin will be further explored. Gas potential in the Bozhong Depression is estimated to be up to 67 Tcf.
The theory of deep buried hill accumulation and the successful exploration practice in the BZ 19-6 field have promoted CNOOC to also explore buried hill traps in other offshore basins in China. Key discoveries have been made since 2019 in the Qiongdongnan and the Pearl River Mouth basins. Additionally, the introduction of new exploration and drilling technology is also key to effectively produce the deeper natural gas in the offshore basins.
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