Subsurface Water - Tool for Petroleum Exploration
Authors: Ostroff A.G. 
Published: December 2007

Abstract
The close association of hydrocarbon accumulations with subsurface waters bas led to the use of properties of these waters as a secondary exploration tool. Both chemical and physical characteristics of subsurface waters have been utilized in the search for petroleum. The movement of subsurface waters bas made a significant contribution to the accumulation of petroleum. In addition to dissolved organic and inorganic constituents, hydrodynamic movement, oxidation-reduction potentials, and water classification systems have been advocated for use in petroleum exploration. Water properties have been utilized in locating permeability barriers, faults, geopressured zones, and unconformities. Examples of water as an exploration tool are given.

Introduction
When the topic oilfield water is mentioned, our first thoughts are usually of a waste fluid that may cause many problems. Some of these that immediately come to mind are those of scale, corrosion, and disposal. Although subsurface water is seldom the primary object of our exploration programs, it is primary object of our exploration programs, it is of primary importance in the accumulation and production of petroleum. production of petroleum. Fig. 1 illustrates the areas of petroleum technology that are influenced by or result from subsurface waters. Starting at the center circle with the origin of water in the subsurface and working out toward the edge, we can appreciate the areas of petroleum technology that are influenced by water. petroleum technology that are influenced by water. Water in the subsurface was important in the hydrocarbon migration and accumulation. Through the processes of dissolution and precipitation, water has influenced porosity and permeability. The chemical environment has influenced the redox potential and the dissolved species in solution. potential and the dissolved species in solution. Trace constituents, hydrodynamics, and classification systems have all been used as exploration tools. It is this portion of the diagram that is discussed in this paper. The following sections of this paper briefly describe the processes that affect subsurface water and relate dissolved constituents to the chemical and physical conditions existing in the subsurface. It is these processes that complicate the use of subsurface water as an exploration tool and the chemical and physical relationships that enable us to use water as a tool. Illustrations of some uses of subsurface water as a tool in petroleum exploration are offered.

SOURCE OF SUBSURFACE WATERS
Water in subsurface formations may be present as a result of either water trapped during sedimentation or infiltrating meteoric water, or a combination of the two sources. Water of sedimentation may range from partly evaporated sea water to nearly fresh water. In this marine or aqueous environment, sediments more dense than water settle to the bottom and collect with the water content highest in the topmost layers of sediments. As the height of sediments increases, compaction decreases the pore spaces between the solid particles and forces the water (and often hydrocarbons) from the sediments.
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