Abstract:

The first member of the Paleogene Funing Formation (hereinafter referred to as E₁f₁) is one of the oil-rich and gas-rich strata in Gaoyou Sag, Subei Basin. It has considerable exploration potential in deep reservoirs, but the overall poor reservoir physical properties have constrained the exploration expansion in this area. To determine the distribution patterns and favorable zones of local "sweet spot" reservoirs with well-developed abnormally high porosity and favorable reservoir conditions, it is necessary to systematically study the genetic types and reservoir evolution models of abnormally high porosity in deep E₁f₁ of Gaoyou Sag. Using reservoir physical property analysis, thin section observation, scanning electron microscopy, and cathodoluminescence, along with stripping the thickness of the Yancheng Formation and dividing tectonic units, this study conducted the physical property analysis of deep reservoirs in E₁f₁. Significant differences were observed in the distribution characteristics of abnormally high porosity among different zones. Specifically, the slope zone developed two abnormally high-porosity intervals, while the fault-step zone developed only one abnormally high-porosity interval. Additionally, the abnormally high-porosity development zones were identified. By analyzing differential genetic controlling factors of abnormally high porosity in different zones, it was determined that oil and gas charging and abnormally high pressure were the two key factors controlling the development zones and genetic types of abnormally high porosity. Oil and gas charging contributed to promoting dissolution and inhibiting cementation, serving as a key controlling factor for the development of abnormally high porosity in the middle slope and fault-step zones. In contrast, abnormally high formation pressure facilitated oil and gas charging, and contributed to diagenetic processes such as promoting dissolution, inhibiting cementation, and slowing compaction, functioning as a key controlling factor for the development of abnormally high porosity in the inner slope zone. Based on differences in key factors and the distribution of formation pressure zones, abnormally high porosity in E₁f₁ was classified into two genetic types: oil and gas charging type and abnormally high pressure type. The oil and gas charging type was mainly distributed in the middle slope and fault-step zones, characterized by relatively shallow burial depths and local occurrence in deep field. The abnormally high-pressure type was mainly located in the inner slope zone, representing the main type in deep field. Two evolution models for abnormally high porosity in deep E₁f₁ were established. Specifically, the middle slope and fault-step zones were mainly characterized by oil and gas charging and pore-preserving dissolution model, while the inner slope zone was characterized by oil and gas charging and abnormally high pressure for pore-preserving superposition dissolution model. On this basis, it is indicated that effective traps in structural high zones overlapping with abnormally high pressure areas of formations in the inner slope and other deep field are favorable locations for the development of “sweet spot” reservoirs, providing a basis for expanding exploration in deep field.

Key words: Subei Basin, Gaoyou Sag, first member of Funing Formation, tight sandstone, deep reservoirs, abnormally high porosity, genetic types, pore evolution models