Structural and geodynamic control of abnormal thermobaric conditions and physical properties of oil reservoirs in the Chezhen sag of the Jiyang depression, East China

Abstract

Geologically, the rift system of East China has a complex block structure influenced by large-scale and differently directed tectonic movements which took place in the Earth’s crust many times in the geological history of East China. The neotectonic movements greatly complicated the geological structure and, naturally influenced formation of the modern fluid dynamical system. Exploration for oil within the Chezhen block of the Shengli field involves certain difficulties associated with the complex block geological structure, small size of most oil pools and varying thermobaric conditions within a pool (even in the range of two neighbor wells). This article describes the studies aimed at enhanced efficiency of exploration for oil, including shale oil which is a new petroleum source in East China. The physical properties of oil and the thermobaric conditions of oil occurrence in the Paleozoic and Paleogene deposits in the Chezhen sag are described. Well information is analyzed in package with geological and geodynamic data. It is for the first time found that in the Chezhen sag: the local areas of increased thermobaric conditions in the oil pools and the better quality oil (lower density and viscosity) occur at the faults of the younger age; the actual deviations from the classical geological and physical correlations (direct reservoir pressure–temperature–depth dependence, direct oil viscosity–temperature/depth dependence) are governed by the migrational geodynamics; the borderline between the notions of a reservoir and a non-reservoir (cap) is quite tentative. The authors propose two mechanisms of origination of the abnormally high reservoir-pressure zones in pools of oil, including shale oil, associated with the migrational geodynamics and with the formation and accumulation of hydrocarbons.