--> Paleogene Igneous Rock's New Model for Hydrocarbon Accumulation in the South Slope of Huanghekou Sag in Bohai Bay

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Paleogene Igneous Rock's New Model for Hydrocarbon Accumulation in the South Slope of Huanghekou Sag in Bohai Bay

Abstract

Under the guidance of geological model, by taking “integration of seismic-geology” as research methods, we make a comprehensive recognition to igneous rock in the south slope of HHK sag in Bohai bay. Through the study of igneous rock eruption stage and features, we identify the igneous rocks' influence on the elements of hydrocarbon accumulation, also by taking the advantage of igneous rocks' features of high-density, we put forward to a pattern of “igneous rock controls trap”, which helps to discover more than 70 km2 of structure trap in the study area where structure traps can't be depicted clearly before. Based on the prediction of tight overflow facies'high block quality, we build the lateral sealing mode by combining igneous rock with small fault, which helps to solve exploration challenge that exploration failure occurs due to the blurred small fault displacement. Recently, based on the new model we drilled the well BZ-X on the target that develops igneous rock, and discovered hundreds of meters oil layer, which is the biggest discovery in HHK sag. Especially, the Paleogene sandbodies, with a depth of over 3000m in this well, has a high porosity with a range of 20% to 30%, which is very rare in HHK sag. Through the observation of reservoir micro-characteristics, and the analysis of clay minerals, like Zeolite, Sodium feldspar and montmorillonite-chlorite mix clay in the reservoir, we establish the evolution pattern of fluid-diagenetic in igneous rock reservoir development area and discuss the control factors of high quality reservoir in this region:1) The igneous rock eruption period matches the diagenetic stage, so the mesh stent structure, formed by plenty of volcanic substance and channels, widely supports the reservoir below them, which makes the reservoir has a lower compaction degree and protects the primary pores. 2) The basalt eruption and leaching changed the whole aqueous medium conditions from terrigenous weak acid fresh water to weak alkaline water in early diagenesis during the Paleogene, which suppressed the dissolution of feldspar, lithic fragment and precipitation of kaolinite in early digenesis. Meanwhile, the local calcite and zeolite began to cement, which helps to protect the primary pore. 3) The area experienced fast hydrocarbon filling in late stage. The aqueous medium conditions changed from alkaline to acid, which leaded to the dissolution of zeolite and calcite and at the same time produced the massive secondary pores.