Hydrocarbon accumulation mechanism and structure o

Authors: CaiNeng Zou LianHua Hou ShiZhen Tao XuanJun Yuan RuKai Zhu JinHua Jia XiangXiang Zhang FuHeng Li ZhengLian Pang

Publish Date: 2011/12/15

Volume: 55, Issue: 2, Pages: 221-235

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Abstract

The Upper Carboniferous in northern Xinjiang China was formed in a postcollisional depression and collapsed structural setting Within the Upper Carboniferous volcanic rocks and source rocks alternate over a wide region At the end of the Carboniferous these layers were uplifted by plate collisions and subsequently weathered and leached Volcanic weathering and leaching led to the establishment of weathered crusts that can be divided into five layers Corrosion and crumble zones in these layers form favorable reservoirs Volcanic weathering crust formed in subaerially exposed paleogeomorphic areas the five relatively continuous layers are generally preserved in paleogeomorphic lowland and slope regions but the upper soil layer is usually absent in structurally higher parts of the rock record The thickness of the weathered layer has a positive nonlinear exponential relationship to the duration of weathering and leaching and the dynamic equilibrium time of weathered crust is about 363 Ma The thickest weathered layer ∼450 m is located in fracture zones Weathered crusts are possible from a range of volcanic rocks with different lithologies given sufficient time for weathering and leaching The combination of volcanic weathered crust and source rocks results in three types of hydrocarbon accumulation models 1 sequences of volcanic weathered crust interbedded with source rocks 2 a quasilayered weathered volcanic core located above source rocks and 3 volcanic rocks associated with pectinate unconformities adjacent to source rocks Each of these three types has the potential to form a giant stratigraphic reservoir of volcanic weathered crust This knowledge has changed the traditional exploration model of searching for favorable lithologic and lithofacies zones in volcanic rocks and has changed the viewpoint that the Carboniferous does not have the genetic potential to be the basement of the basin in northern Xinjiang The concepts developed here are of great scientific significance and application for focusing oil and gas exploration on volcanic weathered crust As such the Paleozoic volcanic weathered crust in the midwestern part of China may possibly contain largescale stratigraphic reservoirs and thus could be a new oil and gas exploration target in the future

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