Abstract: Pre-Tertiary Fractured Reservoirs in the Western Bohai Bay Basin, PR China

WU, CHANGLIN, International Exploration of Kerr-McGee Corporation, Houston, USA

INTRODUCTION

Pre-Tertiary basement reservoirs or buried-hills are major oil and gas plays in the western Bohai basin. Numerous oil fields have been found and exploited since the discovery of the first carbonate buried-hill in the Gangxi Uplift in the early 1960s. These fields include the famous Renqiu oil complex in the Jizhong depression, the well-documented Zhuangxi carbonate fields in the Jiyang depression, and the offshore BZ-28 in the Bozhong depression. According to our statistical data of the oil fields discovered in the western Bohai Bay area in the past three decades, the proved reserves in the pre-Tertiary reservoirs account for around 4-5% of the total proved reserves. It is much lower than the average in the similar basins. This is attributed to the poor quality of seismic data in shallow water transition zones and volcanic rocks in the shallow horizons that made geological interpretations difficult. With more and more drilling data and high-resolution 3-D seismic data, however, we are able to delineate the buried-hills, to separate the Tertiary source horizons from pre-Tertiary reservoirs, and to determine the closure of basement highs. Our integrated studies, mostly rooted in the 3-D seismic and well database from the offshore Bohai basin and onshore Shengli-Dagang oil fields, have indicated that the exploration for pre-Tertiary reservoirs will have a great potential.

Tectonic Setting and Evolution

The Bohai basin is commonly thought to be a Tertiary rift basin in a craton. The tectonic origin of the Tertiary rift basin is closely related to the extension caused by the Western Pacific plate subduction under the Eurasian continent and transtension caused by collision between the Indian plate and Eurasian continent. It was created by mantle doming and continental rifting due to the extension and was also affected by strike-slip faulting. It is actually a composite fault basin sharing many structural characteristics and sedimentary patterns of riff basin and pull-apart basin. This can be illustrated by such distinguishing features as shallow Moho depth of about 29 km at the center of the basin, high subsidence rate, strike-slip faulting, rapid facies change, numerous re-activated faults, asymmetric half-grabens, cyclic sedimentary stacking pattern, and intensive volcanic intrusions.

The Bohai basin also has thick Paleozoic and Mesozoic sedimentary fills, which are considered as basement reservoir rocks. The tectonic origin of the Pre-Tertiary basin is an issue of much debate. Some researchers argue that it is a foreland basin created by regional collision between the Sino-Korean and Yangtze blocks during the Mesozoic. Pre-Cenozoic decollement structures and overthrusts were recognized in many seismic profiles. Some argue that the pre-Tertiary basin is a block-faulted basin developed in tensional settings. This study believes that, during the Jurassic-Cretaceous, the greater Bohai basin was an intraplate rift within the Sino-Korean craton. The suture zone of the Yangtze and Sino-Korean blocks was probably far from the Bohai basin. It is extremely important to define the stress regime of the basin because basement or buried-hill plays rely heavily on the tensional structural elements in terms of a petroleum system. The following structural units are products of tensional regime.

1). Tilted topography or small hills are focal points of oil and gas accumulation;

2). Large faults, usually border faults in the downthrown side of the half-graben, are facilitated as good migration pathways for hydrocarbon;

3). Unconformity surfaces in the gentle side of the half-grabens are commonly karst zones and also good oil carrier beds;

4). Fracturing, minor faulting, and karsting usually happened along large normal faults

Development of the Pre-Tertiary Reservoir Rocks

The greater Bohai basin is a part of the North China Platform which has Archeozoic to Proterozoic basement. Overlying the basement are sedimentary sequences of the upper Proterozoic and lower Paleozoic carbonates which were regionally uplifted and extensively eroded by the Caledonian movement about 460 Ma. Subaerial weathering and erosion of the carbonate platform probably lasted over 100 million years. The hiatus between the middle Ordovician and early Carboniferous was long enough for development of fracturing and karsting. The Carboniferous and Permian continental sediments were only deposited in restricted areas.

By 248 Ma, active subduction occurred within the Laurasia continent. The Sino-Korean and the Yangtze platform collision started. This led to uplifting and erosion of the entire Bohai basin. The hiatus lasted over 40 million years. It is the second stage for development of weathering, fraturing, and karsting. The Indosinian-Yanshan movements about 200 Ma further heavily deformed the Sino-Korean Platform. Local basins were created and had Jurassic-Cretaceous depositions.

The last tectonic activity was the Himalayan movement. A large rifling was initiated by this movement. Intensive fracturing, tilting, and faulting happened in the entire Bohai basin until 55 Ma. This is the third important stage for karsting of carbonates, fraturing, weathering, and leaching of the old granite basement.

Types of the Pre-Tertiary Reservoirs

The pre-Tertiary reservoirs discovered in the western Bohai can be classified into three groups according to lithology: the Cambrian-Ordovician carbonates, pre-Cambrian granites, and Mesozoic volcanoclatsics. The carbonate reservoirs can be subdivided into three types on the basis of stratigraphic contact relationship, fracture density, pore type, and configuration. The reservoir characteristics are summarized as follows.

1). Kerr-McGee-type buried-hill. This kind of carbonate reservoirs is characterized by symmetric host geometry, breccia zone right below the unconformity, tight zone dominated by microfractures, both primary (intergranular) and secondary (fracture) porosity systems, the Dongying-Shahejie shale as seal, 400 Ma hiatus between the reservoir and seal, major faults as migration pathways, abundant minor faults within the reservoirs, and high flow rate in terms of production. The major risks for this kind of reservoir includes minor faults that may be connected with wet zones, downdip limits of karsted zone, and heterogeneous properties. This type of reservoir is the best of the pre-Tertiary oil fields. It includes Kerr-McGee CFD2-1-1,427 field, and Bozhong-28.

2). Zhuangxi-type. It has following features: assymetric tilted hill geometry, a major fault bounding one side and an unconformity bounding the other, both acted as migration pathways, the Permian-Carboniferous or Mesozoic strata as seal in one side, and Shahejie-Dongying seal in the other, relatively short time for karstification due to Paleozoic-Mesozoic deposition, relatively poor fracture-pore systems, and low flow rate in terms of production. The major risks are delineation of karsted zone, poor physical properties, and uncertainty of migration. This type of reservoir includes the Zhuangxi and West Boxing oil fields in the Shengli area, and 428 field in the offshore area.

3). Gubei-type. This type of buried-hill is completely sealed by the Mesozoic or upper Paleozoic strata. The only migration pathway is minor faults. It has the poorest physical properties due to reworking of the upper Paleozoic sediments and mixing of the Mesozoic volcanic dusts. The karsting and fracturing time is also short. This type of reservoirs includes the Yihezhuang and Gubei oil fields in the Shengli area.

4). CFD1-6-type granite buried-hill. The pre-Cambrian granites were highly fractured due to faulting and uplifting of basement. Most of these buried-hills are located on the ramp of paleo-highs or large uplifts. They are usually bounded by two large faults, which are also good migration pathways. Source rocks are directly overlying on the fractured granite. This type of oil field includes the Caofeidian1-6 on the Shaleitian High and Zhengjia fields in the Shenglin area. Major risks of granite reservoirs are difficult delineation of fractured zones and the height of the oil column.

5). Mesozoic-type. The Jurassic and Cretaceous volcaniclastic sandstones are sometimes considered to be basement reservoirs. They are covered directly by Shahejie-Dongying source rocks, therefore the source, migration, and seal are not problems. The reservoir quality relies highly on minor faults and fractures. The volcanic elastics usually have lower to medium porosity and very poor permeability. So far several oil fields have been discovered in the western Bohai Bay, such as Changdi field in the Shengli, 428W from Jurassic sandstones on the Shijiutuo High, and Tang2-2 in the Jidong area Recent discovery was reported from Apache's wells in the shallow water transition zone. They tested 3,000 bbl from Jurassic sandstones and volcanic clastics.

Concluding Remarks

The exploration of the pre-Tertiary reservoirs in the western Bohai has recently attracted a great deal of attention, particularly since the Bohai Oil Company tested 10,000 bbl from well CFD1-6-1 in the Sinian granite and Kerr-McGee tested 8,000 bbl from well CFD2-1-1 from the Cambrian-Ordovician carbonate and Shahejie grainstones. This kind of target has great potential, but it has more complicated reservoirs, and the structural closure is difficult to delineate. It needs more integrated studies for high exploration success. This study suggests the following works be conducted for pre-Tertiary fractured and weathered reservoirs:

1). 3-D seismic should be shot on the ramp of the paleo-highs. It will help determining the downdip limits of possible weathered zones and delineating the entire trap.

2). Structural highs within the depocenters are all focal points of hydrocarbon migration and have potential of commercial accumulations. Analog field studies are needed to help understand the buried-hill reservoirs in the deep part of the basin.

3). It is very important to determine whether the pre-Tertiary reservoirs are basement rocks or buried-hills. Primary study of pre-Tertiary subcrop has shown that the Paleozoic carbonates, pre-Cambrian granites, and Mesozoic sandstones are distributed in several zones in the western Bohai Bay. The carbonates and granites are mainly distributed on the uplifting areas, while the Mesozoic reservoirs are limited in ramps of the uplifts or central parts of the depressions. Detailed mapping of the subcrop of the entire Bohai Bay is recommended.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah