Print this pageAbstract: Prospects of Hydrocarbon Exploration In Bohai Bay
ZAISHEN, GONG, ZHU WEILIN, JIANG WENRONG, China National Offshore Oil Corporation
Introduction
In Bohai Bay Basin, there are 55,700 km2's available offshore area for hydrocarbon exploration. The oil and gas resources are respectively 4 billion tons and 288.7 billion m3. There are totally 228,221 km 2-D seismic profile, 86,736 km 3-D, 48 exploration wells and 122 appraisal wells which have been recorded and drilled during the past more than 30 years since the beginning of geophysical prospecting for oil in 1959. Hydrocarbon has been discovered in Paleogene Shahejie Formation, Dongying Formation and Neogene Guantao Formation and Minghuazheng Formation, also in Pre-Cenozoic limestone, elastic rock and mixed granite. Up to now, we have discovered 14 oil fields, 1 gas field and 49 oil and/or gas-bearing structures, with oil reserves in place of 750 million m3. The undiscovered hydrocarbon resources are more abundant, which shows bright prospects in oil exploration.
Geologic Settings For Oil Enrichment
Multiple oil and gas-bearing formations and trap types were documented by oil exploration survey in Bohai Bay Basin. Each depression has undergone Paleogene rifling and Neogene post-rifting stages, but these depressions separated from each other in the early period, the structural evolution histories in these depressions are different, and so to the richness of hydrocarbon generation. Therefore, the unique geologic settings in different depressions are the key for oil enrichment:
1. Rich oil-generating depression is the material foundation for giant oil and gas reservoirs
The primary source rock in Bohai Bay Basin is middle-deep lacustrine mudstone in Shahejie Formation with resources of 75 billion tons, the secondary one is mudstone of lower Dongying Formation. With geochemical study and basinal modelling analysis, we could tell that there are 14 depressions in Bohai Bay have certain potentialities in hydrocarbon generation. Among them, 3 depressions ( Liaozhong, Bozhong and Chikou ) are grade 1, 4 ( Bodong, Huanghekou, Laizhouwan and Shanan ) grade 2. The average about the richness of resources is about 150,000 t/km2, and belongs to rich oil-generating depression ( Fig. 1 ). Especially in the followed three depressions: Liaozhong, Bozhong and Chikou, the thickness of Shahejie Formation is 5,000-6,500 m, that of Dongying Formation is 2,000-3,600 m. The primary source rock-member 3 of Shahejie Formation has been buried beneath 3,000-5,000 m, the content of organic matter is 0.68-1.66%, total hydrocarbon is 411-1,680 ppm, the S1+S2 is 2.17-6.49 mg/g, the kerogen is mainly type II, and the oil threshold is between 2,500 m and 3,300 m. The most of source rock has entered in the mature and high mature stages, only the lower part of member 3 of Shahejie Formation in Bozhong depression has entered in overmature stage.
In the onshore area, some medium-giant sized oil fields have been discovered around the rich oil-generating depressions. For examples, 6 big oil fields have been discovered in the western depression of Liaohe sag which is close to Liaodong bay. The 6 oil fields are Shuangxiling, Shuguang, etc., with proven reserves in place of 1.15 billion tons. 5 big oil fields such as Gudao, Gudong, Wuhaozhuang, etc. have been discovered in Zhanhua depression in west of Bozhong depression, with proven reserves in place of 870 million tons.
2. Regional structural highs are the favourable site for big oil fields
Big nose arch on the slope zone, submerging sloping mountain on old uplifts, horst in depressions and faulted steps are the main directions of oil migration for the rich oil-generating depressions. For example, Shuguang, Shuangxiling and Gaosheng oil fields are located on the nose structures of slope zone, the types of oil reservoir are unconformity, faulted block, rollover anticline, sandstone pinchout and buried hill; Bozhong 3-1 oil field is also a complex oil field with different types of trap in Minghuazheng Formation, Guantao Formation, lower Dongying Formation and the basement on a nose structure of an old uplift, with proven reserves of 50 million tons; Suizhong 36-1 oil field is a drapal anticline in Dongying Formation on an uplift, with oil reserves in place of 260 million tons and gas of 220 million m3; Bozhong 34-2 oil field is a faulted anticline on a uplift zone with richness of reserves over 2 million tons/km2.
3. Long-term activities of fault is favourable for oil migration and accumulation
The major faults developed in Paleogene had mostly but not strongly rejuvenated with different degree in Neogene, the fault throws are generally about 100 m, and end in Pliocene Minghuazheng Formation. The activities of fault not only connects between source rock in deep part of the depression and sandstones in shallow part and high places to provide a good migration pathway for oil lateral and vertical migration, but also creates major sites for oil accumulation in Bohai Bay Basin, with reserves occupying 58% of the total in basin, as the strong fault activities in Paleogene resulted in the structural framework alternating horsts with grabens and continued to develop drapal structures on horsts and rollover anticlines along faults in Neogene by the effect of filling, compaction newly-rejuvenated fault activity.
4. Large volume of reservoir is the key for the formation of big oil fields
The main reservoirs in Bohai Bay Basin are Minghuazheng-Guantao Formations, lower Dongying Formation, Shahejie Formation and pre-Cenozoic buried hills.
a. Large deltaic sandstone in Paleogene is good reservoir in lower Dongying Formation and Shahejie Formation. For examples, the hundred-million-ton sized oil fields. Shentuo and Dongxin, located in the central-north part of Dongying depression, have the deltaic sandstone in member 2 of Shahejie Formation as main reservoir. The sandstone is 20-80 m thick, with porosity of 20-30% and permeability of 1-10 µm2. Suizhong 36-1 oil field is located on the Liaoxi uplift, its reservoir is the deltaic sandstone in Dongying Formation, with total thickness of 900 m and porosity of 31-33% and permeability of 0.2-0.6µm2.
b. The Neogene fluvial sandstone is excellent regional reservoir. The strata in lower Minghuazheng Formation-upper Guantao Formation are regionally intercalation of sandstone and mudstone with unequal thickness, resulting in the most favourable interval for oil accumulation. For examples, the pay zones in Gudong and Gudao oil fields are generally about 10 m thick, with porosity of 32-35% and permeability of 1-1.5 µm2.
c. The pre-Cenozoic basement reservoirs are mainly the carbonates of Lower Paleozoic-Middle to Upper Proterozoic and Archeozoic metamorphics.
Great Prospects of Oil Exploration Exist In Bohai Bay
1. Large sums of potential resources
Bohai Bay has been divided into 15 depressions and 13 uplifts, the total area of depressions is 37,650 km2. The discovered traps more than 2 km2 in area are summed up to 352, with total closure area 3,746.2 km2. Among them, 86 objects have been drilled, 14 oil & gas fields and 49 oil-bearing structure have been discovered. Only 3 big oil fields: Suizhong 36-1, Qinhuangdao 36-2 and Nanpu 35-2 are discovered up to now. Therefore, the discovered oil fields in offshore area are much less than its adjacent oil province not only in quantity but also in reserves, which illustrates great potentiality for oil exploration exists in Bohai Bay.
It is worthy to mention that the total calculated resources is 450 million tons in three depressions: Huanghekou, Chengbei and Shanan but the oil reserves of 740 million tons, much bigger than the calculated resources have been discovered in Chengbei uplift which is surrounded by these three depressions.
2. With wide fields for exploration
a. With a series of favourable uplifts for oil accumulation
The strong fault activity in Paleogene creates the structural framework with many uplifts and depressions, one structural high may be surrounded by one or more depressions. In addition, the provenance are the margins of depression and structural highs between depressions, the short rivers results many sandstones which can provide favourable space for oil accumulation. For examples, these uplifts are Liaoxi, Liaodong, Bodong, Bonan, Shaleitian and Shijiutuo (Fig.2).
b. With several reservoir-seal associations
Drilling in Bohai Bay illustrates that there are 5 reservoir-seal associations respectively in lower Minghuazheng Formation-Guantao Formation, Upper Dongying Formation, lower Dongying Formation, Shahejie Formation and pre-Cenozoic basement.
The first, third and fourth associations are regionally distributed, their reservoirs are mainly fluvial, deltaic and fan-deltaic. But different areas have different horizons of oil enrichment in Bohai Bay Basin. For example, in Liaohe sag the main target horizon is Shahejie Formation in which the discovered reserves occupies 65% of the total reserves in the sag; The reservoirs in Huanghua sag are in Neogene Minghuazheng-Guantao Formation and Paleogene Shahejie and Kongdian Formation, with 87% reserves of the total; The main reservoirs in Jiyang sag are in Minghuazheng-Guantao Formations and Shahejie Formation, with 93% reserves of the total. Whereas the discovered reserves in Dongying Formation in offshore area are 80% of the total. Therefore both Neogene and Paleogene formations have large proportion of reserves in the onshore explorational area of Bohai Bay Basin, and the Paleogene hydrocarbon concentrates in Shahejie Formation, not in Dongying Formation, which is different to that in offshore area. These reflect two aspects of thing: one is that the explorational degree is still low in offshore area and new fields are existing for further exploration, another is that the geologic settings are different between onshore and offshore areas. There are very thick Paleogene Dongying Formation in Bozhong and Liaozhong rich oil-generating depressions, the thickness of it is 2000 m thick than in onshore area, which introduces the following topics.
3. With shallow oil resources
There are two tectonic sedimentary cycles in Paleogene in Bohai Bay: Eocene rifting stage and Oilgocene faulting-subsidence stage. Deep lacustrine mudstone of member 3 of Shahejie Formation was deposited in the active rifling period and middle-deep lacustrine mudstone of lower Dongying Formation was deposited in the flourishing period of faulting-subsidence stage. The thickness of lower Dongying Formation is generally 2000-36—m thick in Bozhong and Liaozhong depressions, the mudstone in it has organic carbon content of 0.94-1.47%, total hydrocarbon of 297-607 ppm and S1+S2 of 0.85-5.04 mg/g, the kerogen is mainly type II. Most of the source rock is in mature or low mature stage, showing good explorational potentiality in shallow horizons.
Exploration Directions
1. South faulted-step zone in Shijiutuo uplift
The downthrown side of south faulted-step zone in Shijiutuo uplift, i.e., the slope zone in north flank of Bozhong depression, is a big nose structure. There are a series of Neogene rollover anticlines and faulted blocks, the hydrocarbon minrate through growth faults from deep to concentrate in these traps. The well BZ 3-1-1 indicates that high productive oil and gas flows come from Minghuazheng Formation, lower Dongying Formation and buried hill.
2. Bodong uplift
There are drapal anticlines developed on Bodong uplift and rollover anticline on the downthrown side in west of the fault. Oil and gas flows have been discovered in Minghuazheng Formation in Liaodong 32-2, showing good explorational prospects.
3. Shaleitian uplift
This uplift had exposed above lake level for a long time before Neogene, and been surrounded by four depressions with deep lakes developed in them: Bozhong, Shanan, Qikou and Nanpu, and it also was a major provenance for the basin. In the period of deposition of Shahejie and Dongying Formation, there were many alluvial fan and subsaqueous fan around it. With assorted sgnsedimentary anticline and drapal faulted nose on basement, it possesses the general conditions to result in large oil fields.
4. North part of Liaoxi uplift
The Liaoxi uplift is located between two oil-generating depressions: Liaozhong and Liaoxi, deltas and nearshore subaqueous fans developed along its two sides in north part. Especially in the east slope zone, the delta-front sand bodies pinchout or lateral lithologic changes usually happened along the updip direction to make stratigraphic traps, having the geologic settings for the formation of large scale oil reservoirs.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah