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 weathered products. 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, whereas 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 weathering and leaching, but its permeability is a function of clay mineral content, i.e., the higher the clay content, the worse the permeability. Weathered crust provides effective sealing conditions for both top and bottom layers of a petroleum reservoir, and is important in the clastic hydrocarbon exploration. structure of weathered crust, weathering index, Junggar Basin, reservoir properties, hydrocarbon exploration Citation: Zou C N, Hou L H, Yang F, et al. 2014. Structure of weathered clastic crust and its petroleum potential. Science China: Earth Sciences, 57: 3015 -3026,

Reservoir variations and heterogeneities under valley‐monadnock palaeo‐geomorphology in hinterland area of the Junggar Basin, northwestern China: Sedimentary characteristics and diagenetic alterations

“…The formation of the weathering crust around the monadnocks was distinctively accompanied with the leaching and migration of the chemical elements via rainfall and groundwater. Generally, the alkaline earth metal elements (i.e., K, Na, Ca, and Mg) have the strongest migration ability and can be leached out in quantity (Fedo, Nesbitt, & Young, 1995; Parker, 1970; Zou et al, 2014). In contrast, the Fe element has the medium migration ability, whereas the relatively stable Si and Al elements are usually enriched in situ (Lybrand & Rasmussen, 2014; Zhang et al, 2017; Zou et al, 2014).…”

“…Generally, the alkaline earth metal elements (i.e., K, Na, Ca, and Mg) have the strongest migration ability and can be leached out in quantity (Fedo, Nesbitt, & Young, 1995; Parker, 1970; Zou et al, 2014). In contrast, the Fe element has the medium migration ability, whereas the relatively stable Si and Al elements are usually enriched in situ (Lybrand & Rasmussen, 2014; Zhang et al, 2017; Zou et al, 2014). Particularly, the leaching and migration activities of the alkaline earth metal elements can locally change the pore‐water chemistry into alkaline environments (Garcia, Morad, Ros, & Al‐Aasm, 1998; Haile et al, 2018; Morad et al, 2000).…”

Quantitative identification of triassic top unconformity: vertical structure and its reservoir significance in the sanbian area of the ordos basin

“…In terms of the physical properties, the porosity and permeability of the weathered leaching zone are higher than those of the residual layer above the unconformity and the unweathered original rock below the unconformity. The porosity and permeability decrease sharply near the weathered clay layer (Zou et al, 2014).…”

“…A clastic unconformity is generally composed of a residual layer (e.g. a bottom conglomerate or water-bearing sand body) above the unconformity surface and a paleo-weathering crust (subdivided into a weathered clay layer and a weathered leaching zone) below the unconformity surface (Wu et al, 2003; Song et al, 2010; Zou et al, 2014). At present, there are four methods for identifying unconformity structures in clastic rocks.…”

“…The second method is quantitative analysis conducted based on limited petrophysical property testing and rock element contents (Zou et al, 2014); the element content method can be divided into the trace element method and the major element weathering index method (Zhao et al, 2009). The third method is restoration of the paleogeomorphology of the unconformity by means of the impression method and the residual thickness method (Song et al, 2003; Guo and Sun, 2006).…”