Genesis Analysis and Development Technology for the Marine Sandstone Reservoir with Low Permeability and Resistivity in Western Pearl River Mouth Basin, South China Sea

Zhenfeng Wang, Yingzhao Zhang, Daojun Zhang, and Zhaoqiang Yang
CNOOC Ltd.-Zhanjiang, Guangdong, China

Development for the oilfield with low permeability and resistivity is one of the difficult problems. With the advancement of exploration and development technology, oil production in these reservoirs accounts for an increasingly large proportion of total output. A number of marine reservoirs with low permeability and resistivity are discovered in the Western Pearl River Mouth Basin, South China Sea due to recent years’ continuing exploration efforts. The above mentioned reservoirs, characterized by shallow burial, widespread distribution, unconsolidated lithology, fine grain size, high shale content, low permeability and low resistivity, high initial water saturation, couldn’t be applied Acid-Fracturing Technique to enhance oil-well productivity. Fine grained reservoirs result in a series of problems: difficult logging parameter evaluation, abrupt lateral variation of effective thickness of reservoir, confused reservoir physical property distribution, complicated hydrocarbon detection, hard determining stable productivity and reasonable development program, and difficult matching well drilling and completion technology. Therefore, further study for the genesis of low resistivity and low permeability reservoir and related development technology has a very important significance to hydrocarbon exploration and development in northern South China Sea.

Integrated analysis of drilling, core, logging, seismic data shows that fine gained lithology, high shale content, large ratio of fine–micro pore volume, resulting in high bound water saturation in these reservoirs, which is the genesis of the low resistivity and low permeability of marine reservoirs in Pearl River Mouth Basin. According to foraminifera distribution, the reservoir were deposited in outer shelf environment and palaeotectonics controlled the scale and physical property of argillaceous siltstone reservoirs of neritic shoal-bar environment. Comprehensive study shows that, exploration potential intervals of low resistivity and low permeability reservoir are distributed on the top of high resistivity and high permeability reservoir of Zhujiang Formation, in which some oil fields have been discovered in uplift area of Pearl River Mouth Basin, and high hydrocarbon intensity and tectonic filling intensity are the primary reasons for the formation of low permeability and low resistivity reservoir. These low resistivity and low permeability reservoirs are mainly located in Qionghai Lower Uplift of Zhusan Sag and Shenhu Uplift, and have potential oil reserves of more than 50x106 m³, which should become an important domain for oil exploration, stabilizing and increasing production in the near future.

By means of combined study to different specialties such as logging, seismology, geology, reservoir, drilling and completion technique, a series of techniques for these reservoirs have been established:
1. BP neural network, wavelet transform of logging identification of low resistivity reservoir, and "three waters model" of the reservoir parameters for quantitative evaluation;
2. Seismic spectral factorization technology, inversion of reservoir distribution, and oil-bearing property prediction technology;
3. long horizontal wells + "sweet spot" of improving the capacity;
4. Effective sand control technique for unconsolidated sandstone-low permeability reservoir.

These techniques have been applied effectively in the exploration and development for low permeability and low resistivity marine sandstone reservoirs in Wenchang area of Pearl River Mouth Basin, which great contribute to new hydrocarbon discoveries, reservoir exploitation and deployment of adjustment wells, and achieve a desired result of new geological oil reserves of 30x106 m³.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013