Delineation Of Trends In Reservoir Quality

Abstract The diagenetic alterations of sandstone occurs in a continuous system. As a result, equilibrium thermodynamics cannot be strictly used to describe the equilibrium composition of the diagenetic systems. However, when the residence time of the fluid phase (T = V/q) is sufficiently large relative to the appropriate time scale of the diagenetic reaction, the time invariant condition of a continuous system approaches chemical equilibrium and equilibrium thermodynamics are applicable. The residence time is a function of the fluid flow rate through the sandstone and as a result it is possible to have different diagenetic reactions occurring in different zones of a sandstone due to local changes in flow velocity. Diagenetically altered sandstones can be divided into two groups according to the degree to which steady-state approximates chemical equilibrium. In those systems where chemical equilibrium is not closely approximated, the water chemistry is determined by the mineral composition of the sandstone. Understanding this control on sandstone diagenesis is important in delineating zones of different quality and sensitivity in sandstone reservoirs. Introduction The sands which eventually become petroleum reservoirs are initially deposited as an accumulation of chemically unstable grains having high porosity (35–45%) and very high permeability (1–100 porosity (35–45%) and very high permeability (1–100 darcies). As these sands are buried and lithified to become sandstone, the unstable grains are dissolved and more stable minerals are precipitated and the rock as a whole approaches chemical stability. The most common of the new minerals precipitated during burial (diagenetic minerals) are clay minerals. This is one of the reasons that porosity is only slightly reduced while permeability is reduced several orders of magnitude. The clay minerals have very high surface areas (Table I) and as a result, sandstones with diagenetic clay minerals developed within their pore systems have very high surface area to pore volume ratios (Table II). The high specific surface area of the diagenetic clay minerals is the main factor causing the great reduction in sandstone permeability in much the same way that baffels reduce the flow through a tube. These diagenetic minerals are also primarily responsible for the various reservoir primarily responsible for the various reservoir sensitivities (Almon). Because the diagenetic minerals precipitated within a sandstone are the primary agents in the destruction of reservoir quality and in the development of reservoir sensitivities, it is extremely important to understand the factors that influence their development. The question to be answered is: "why do diagenetic minerals develop as they do and is there any way to predict the kind and amount of mineral that has developed in a particular part of a reservoir?" GEOLOGICAL EXAMPLE The Upper Cretaceous (Campanian) Mesa Verde Formation of northwestern Colorado (Fig. 1) is a low permeability gas-bearing, "shaly" sandstone which has been a substantial exploration target for the past few years. Much of the shaliness of these sandstones is due to the development the diagenetic minerals kaolinite, illite and mixed layer illite/ montmorillonite. The degree of development of these diagenetic clays and their influence on reservoir productivity is a major uncertainty in exploring for productivity is a major uncertainty in exploring for and developing such tight gas sands. Two distinct reservoir types are present in the Mesa Verde Formation (Fig. 2). Type I reservoirs were formed by minor fluvial channels which deposited pods of poorly sorted very fine-grained, silty, argillaceous sandstone. Mean grain size in Type I reservoirs varies between 0.08 and 0.14 mm. Porosity averages 12.2 percent and permeability Porosity averages 12.2 percent and permeability averages 1.20 md. Illite or mixed layer illite/ montmorillonite and kaolinite are the major diagenetic clays. Silica, siderite and/or dolomite cements are also developed (Fig. 2).