Genesis of Fine-grained Reservoir and its Significance on Gas Exploration in the Yinggehai Basin, South China Sea

Weilin Zhu¹, Zhenfeng Wang², and Jianxiang Pei^2
1CNOOC, Beijing, China
²CNOOC Ltd.-Zhanjiang, Guangdong, China

The Yinggehai Basin (YGHB), one of important gas-enriched province in South China Sea, is characterized by fine-grained reservoirs, including siltstone, fine-grained sandstone and argillaceous siltstone. It is estimated that about 2 trillion cubic meters of gas were in place in the centre of the basin. Therefore, it is important to reveal the genesis and distribution of fine-grained reservoirs in the YGHB. In the recent years, a couple of gas fields have been discovered in the shallow section (Yinggehai formation of Pliocene) of the mud-diapir structures in the central depression, such as DF1-1, LD22-1 and LD15-1 gas fields. The reservoir consist mainly of fine-grained sediments due to the long distance transportation from the provenances and mostly formed in platform sandstone, tempestite and distal submarine fan in neritic-bathyal environments. Those reservoirs at shallow section(less than 1500m) have weak diagenesis and good-quality physical properties. However, those fine-grained reservoirs at the mid-deep buried strata with a depth about 3000m are dominated by low permeability, as well as high temperature and high pressure (HTHP) because of compaction. It is difficult to exploration operation in those fine-grained reservoirs due to high economic threshold in marine environments. Hence, it is the key to explore sweet spot at the mid-deep buried strata.

Based on the integrated study of sequence stratigraphy, seismic sedimentology, reservoir geology, logging sedimentology, a series of techniques are developed to search the "sweet spots". (1) Large continental marginal delta and submarine fan drainaged from the Lam River in the Kunsong uplift has been identified on the basis of facies markers, paleontology, palaeo-water depth and palaeoenvironment. (2) Based on analysis of fourth-order sequences and reconstruction of paleotopography, the submarine fans at the Dongfang area are described in detail. And multiple episodes and distribution of submarine lobes are also described. Heterogeneity of the reservoir and “sweet spots” have been predicted in space. (3) The mechanism of diagenetic heterogeneities is revealed. Compaction takes the primary role on reduce of the porosity. Anomalous high porosity in the mid-deep strata is a result of channel sub-facies, prevent of overpressure and secondary pores. Moreover, reservoir with higher permeability is present in the region relatively depart from HTHP seal because of weak cementation. (4) A set of multi-attributes fusion of seismic slice analysis technology is proposed according to interactive interpretation of horizon and strata slice, which helps to accurately describe the channel of submarine fan and to identify “sweet spots” under the control of the main channel and branch channel facies. (5) Multi-scale seismic information stripping technology is established to predict quantitatively the physical property of reservoirs and gas content. (6) Based on the lithologic analysis of cores as well as logs data, the reservoirs have been classified and evaluated so that the precision for their physical properties has been greatly improved.

Consequently, the high quality reservoirs in mid-deep buried strata of the YGHB have been identified. Combined with the achievement of pool-forming dynamics pattern, a new play is discovered in mid-deep buried strata of the Dongfang area, which leads to the transformation of exploration strategy from diapir anticline structure in the shallow section to lithologic trap of Huangliu formation in the mid-deep section. A great breakthrough of gas discovery was made in recent exploration in fine-grained reservoirs at mid-deep buried strata, the hydrocarbon-enriched and overpressured gas field DF13-1/2, whose reserve is up to over 100 billion cubic meters. The fact indicates that gas could accumulate in the fine grained section under the condition of high temperature and overpressure in the deep part of the YGHB, which strived into a new target realm for future exploration and has great significance for the gas accumulation theory, especially for the gas exploration in the YGHB.

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