Structure of Weathered Clastic Crust and its Petroleum Potential

Abstract

Weathered clastic crust can be subdivided into weathered clay and leached zone in terms of variable weathering of different minerals and mobility of weathering products. The Junggar Basin, situated in Western China, has experienced extrusion and collision by the Siberian, Kazakhstan, and Tarim plates during the Hercynian, Indosinian, Yanshanian and Himalayan movements. It exhibits 13 regional unconformities and several sub-unconformities which is perfect for researching clastic unconformities. On the basis of clastic outcrops and well cores in the Junggar Basin, the dark red Fe-rich weathered clay is formed in an arid environment, while the light blue Al-rich weathered clay under humid conditions. According to the geochemical analysis, a new weathering index for weathered clastic crust is built mainly on Fe and Al contents, accurately indicating the weathered clay, sandy leached zone, and muddy leached zone in the Junggar Basin. The breaking pressure of weathered clay is rather large, the same as that of normal muddy cap, effectively to seal oil or gas. The porosity of underlying leached zone is greatly enhanced by due to weathering and leaching, but its permeability is a function of clay mineral content, i.e., the higher the clay content and the worse the permeability. Weathered crust provides effective sealing conditions for both top and bottom layers of a petroleum reservoir, and deepens the clastic hydrocarbon exploration. The structure of weathered clastic crust has an important theoretical value and scientific significance to clastic stratigraphgic reservoirs and deep petroleum exploration.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016