Selective Charging of Neogene Sand Bodies in Meandering Fluvial Systems in the Bohai Bay Basin, China

Abstract

Neogene strata of the Chengdao area of the offshore Bohai Bay Basin in China, form a meandering river sandstone reservoir that hosts large accumulations of hydrocarbons. A key exploration issue is that the stacked reservoir sands are variably oil-bearing, although the porosity and permeability of all sand bodies are similar. For this reason, the failure rate of drilling for hydrocarbons in the Chengdao area is up to 50%. In this study, the factors which control hydrocarbon charge were analyzed using geological statistics and physical simulation experiments. The volumetric fill degree was quantified in eighty traps allowing a predictive filling model to be proposed based on multivariate statistics.

The results show that charge and degree of fill are controlled by four factors: (1) The proportion of oil-bearing sand bodies is higher in areas closer to faults, especially those with high transmissivities; (2) The scale of meandering channels affects the connectivity between sand bodies and faults, so that sand bodies in large-scale channels are more likely to be charged; (3) Physical simulation results show that hydrocarbons preferentially fill stratigraphically higher sand bodies when filling is episodic, but preferentially fill sand bodies with better porosity and permeability when filling is continuous; (4) The dip of the sand also affects the probability of changing, but the thickness of the sand body has no effect. With these results, configurations of faults and channel sand bodies were classified into four types according to the characteristics of fault activity and channel scale. Using multivariate linear regression analysis, the dominant controls on the degree of fill were analyzed and four equations constructed to predict the degree of fill. This research provides a quantitative, predictive guide for exploration within fluvial sandstone reservoirs in shallow strata.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019