Development of Secondary Porosity in Reservoir Sandstones by Dissolution of Silicate Mineral Constituents: ABSTRACT

Abstract

Although considerable pore space in reservoir sandstones may result from chemical removal of carbonate rock as cement or mineral replacements, significant porosity may develop by direct dissolution of silicate mineral constituents. Feldspar and volcanic rock fragments, in particular, appear highly susceptible to dissolution in many reservoir sands. Dissolution of detrital feldspar represents a most important source of secondary porosity. Sanidine, orthoclase, and plagioclase occurring as discrete grains or constituents in rock fragments may undergo intense dissolution. Microcline and authigenic feldspar growths, however, are generally much less affected. Early unaltered carbonate cement (pre-feldspar dissolution) in some sandstones indicates that dissolution may occur under alkaline as well as acidic conditions. Although resultant pores may be moldic, most voids contain remnants of undissolved material. In feldspathic sandstones, extensive dissolution can cause weakening and subsequent readjustment of the detrital framework. Dissolution of volcanic rock fragments (VRFs) may also account for considerable pore space in reservoir sandstones. Finely crystalline volcanic fragments and volcanic glass appear to be especially prone to dissolution. Voids resulting from dissolution of VFRs are commonly moldic appearing and may be characterized by a skeletal framework of straight to curvilinear shards and veinlets(?) resembling chert. In some specimens, chloritized VRFs appear to be more susceptible to dissolution than unaltered fresh-appearing fragments. As chlorite is relatively soluble under lower pH conditions, this possibly indicates that formation waters attending dissolution were somewhat acidic. Although dissolution of silicate mineral constituents results in increased porosity, it does not necessarily result in enhanced permeability. In some places, the development of authigenic clay as a dissolution product may serve to constrict pore throats, resulting in a decrease in pore communication. End_of_Article - Last_Page 737------------