Studies of Ventura Field, California, II: Lithology, Compaction, and Permeability of Sands

Abstract

ABSTRACT Permeability of reservoir beds generally decreases in progressively deeper producing zones of the Ventura field. Petrographic studies indicate that the reduction of pore size in sands resulted chiefly from compaction by stress under load. The degree and trend of compaction of the Ventura field sediments have been evaluated by statistical correlations of sedimentologic data (grain size and sorting) and reservoir-engineering data (porosity and permeability). The data permitted the derivation of an empirical relation relating permeability to lithology and compaction: k=Cdm 2e-1.31σϕ, where k is permeability in millidarcys, C an empirical number, dm the medium size in millimeters, and σϕ the sorting expressed in ϕ—standard deviation. Two new concepts are introduced. The granular parameter G is defined as G=dm 2e-1.31σϕ. G gives a measure of the suitability of a clastic sediment as a potential reservoir bed before any diagenetic change has taken place. The empirical number C gives an expression of the degree of compaction. For any given sediment subjected to compaction, the granular parameter should remain unchanged if there has been no significant fracturing. The decrease of permeability is directly proportional to decrease of C. We thus might designate the ratio C to the Co for unconsolidated sediments as the compaction index. For any given region, a knowledge of the depositional environments of the sandy sediments should give a general knowledge of the range of granular parameters. After the degree of compaction is defined by obtaining empirically a value for compaction index, the permeabilities of types of reservoir sands not yet encountered in explored zones can be estimated.