Geology of Gu-Dao Oil Field and Adjacent Areas: ABSTRACT

Abstract

Gu-Dao oil field is located geologically in the center of Zhan-Hua basin, Bohai Bay hydrocarbon-bearing province, and geographically in the coastal zone of the lower Yellow River valley. Zhan-Hua basin is a Cenozoic block-faulted basin, with an area of 2,100 sq km. The intensive subsidence of the faulted blocks was followed by deposition of thick Tertiary continental strata (the Eocene series itself is as thick as 4,500 m), at a sedimentation rate of 0.127 mm/year. The strong block faulting in the basin led to the formation of buried Paleozoic hills, which controlled many overlapping Tertiary structures. Almost 90 of the known oil reservoirs are in these structures. Gu-Dao oil field produces from one of the late Tertiary overlapping structures which is cut along the north and south side by two faults striking nearly east-west. These faults controlled the origin and the development of the comparatively intact structure. A long period of faulting caused vertical migration of large quantities of oil and gas, and the formation of a series of multiple pays. The hydrocarbons are distinctly zoned. That is, in the Paleozoic and lower Tertiary rocks, they are high-paraffin crude oil of high wax and low sulfur content. The upper Tertiary rocks contain highly viscous aromatic-cycle alkane crude oil of low wax, high sulfur content, and dry gas occurs in the upper part of the Neogene and Quaternary. The analysis of the data shows that hydrocarbons are derived from the same source rock--the lacustrine lower Tertiary Sha-He-Jie formation--distributed in the depression surrounding the Gu-Dao structure. A secondary reservoir was formed along the faults during the multiple tectonic movement during the late Tertiary, as a result of the upward migration of hydrocarbons from lower Tertiary rocks. Hence the Gu-Dao oil field is a combination of both primary and secondary reservoirs. The main oil-bearing formation in the Gu-Dao oil field, the upper Tertiary Guan-Tao formation, comprises a set of fluvial deposits, with channel sand bodies as the principal reservoirs. The reservoir extends along the long axis of the Gu-Dao structure, and constitutes the major producing area. The oil field was discovered in 1968 and full production was begun in 1971. The development policy of water flooding at an early stage, either internally, separately, or quantitatively, was adopted according to reservoir characteristics of high viscosity, sand-out, or differences in pays. A series of measures has been taken to control sand and to reduce viscosity. Although 7 years have elapsed since the beginning of production, present oil production per well is equivalent to that of the early stage, a rising trend in productivity is obvious, and good development effects are insured. End_of_Article - Last_Page 698------------