Abstract:

The “new source, old reservoir” type buried hills are an important exploration target in the Dagang Oilfield, Hunanghua depression where an exploration breakthrough is made recently. Controlled by multi-stage fault evolution this type of buried hills is diverse in reservoir types and hydrocarbon accumulation conditions. In this study, the static and dynamic characteristics of the hill-controlling faults are systematically analyzed, and the fault control on the formation of buried-hill reservoirs is revealed. Based on a systematic analysis of the influencing factors in reservoir formation, such as physical properties/reservoir-forming mechanisms of Paleozoic carbonate and clastic reservoirs, source-rock properties, and source-reservoir configurations, the buried-hill types are delineated, and the potential exploration strata and distribution area for each type are identified. Results show (1) the current fault system has the multi-direction, multi-property characteristics resulted by multi-stage superposition evolution. The fault system develops through four main stages: near-EW and NW (W) reverse faults of T3 period continue development in J₁₊₂; NNE (near-SN) thrust-transition fault forms by extrusion thrust in J₃+K₁; the above thrust-transition fault undergoes negative inversion in K₁; and NE (E) extension fault forms in the Cenozoic E period. (2) The buried hill experienced “two-stage extrusion, one-stage block faulting” orogenesis, and the inner reservoir experienced multi-stage diagenesis (early-, supergene, middle- and late-diagenesis). The Ordovician carbonate reservoir has medium porosity/permeability, and the clastic reservoir has medium-low porosity/permeability. (3) With high-quality source-rock developing respectively in the 2^nd and 3^rd members of the Kongdian and Shahejie formations, source-reservoir configuration becomes an important factor controlling the formation of buried-hill reservoirs, and five source-reservoir configuration types are formed under fault control. (4) According to source-rock property, source-reservoir configuration and internal structural style, the “new source, old reservoir” type buried hills are divided into three categories and seven subcategories. The buried hills have a ring-shaped planar distribution, and the potential exploration area includes three zones: Ordovician strata in the multi-stage structural superposition area; middle-low strata in the basin margin; and deep strata connected to the high buried-hill source rocks outside the basin.

Key words: fault features, evolution process, reservoir-forming mechanism, source-reservoir relationship, division of buried hills, Huanghua depression