An attempt has been made to establish reference intervals in the subsurface based on apparent systematic interlayer water loss of swelling clay minerals. The intervals are used in much the same way as the familiar indicators for metamorphism but occur at sufficiently shallow depths to be evident within oil-bearing strata of the Gulf Coast. The resulting conclusion is that clay-mineral diagenesis indicators may prove to be important petroleum-evaluation markers as well as fundamental properties of sedimentary basins.

In this study, sedimentary deposits are viewed as combinations of gases, liquids and semi-solids distributed through a solid matrix. During geologic development, the interstitial components segregate by migration and produce various commercially exploitable concentrations. Water, the principal fluid component of the sedimentary section, is thought to migrate in three separate stages. Initially, pore water and excessive (more than two) clay-water interlayers are removed by the action of overburden pressure. This initial water flow (which is essentially completed after the first few thousand feet of burial) reduces the water content of the sediment to about 30 per cent, most of which is in the semi-solid interlayer form. A second stage of dehydration is thought to occur when the heat ab orbed by the buried sediment becomes sufficiently great to mobilize the next-to-last water interlayer in an M (H₂O)_x + ^DgrH_r = I + xH₂O fashion. The final stage of sediment dehydration which extracts the last remaining water monolayer from clay lattices is apparently slow, even by geologic standards, requiring tens or possibly hundreds of millions of years, depending upon the geothermal and burial history of the sample.

Petroleum hydrocarbons which are distributed throughout the matrices of potential source beds in normal frequencies of 300-3,000 ppm. are thought to be too sparse to initiate continuous fluid flow. In normal marine sediments, however, the water associated with clay minerals is present to a considerable depth in the order of 200,000 ppm., and it is therefore reasoned that this phase forms the connection between petroleum source and reservoir beds.

The first and last dehydration stages probably are unimportant in Gulf Coast oil migration inasmuch as they occur at levels which are too shallow or too deep, respectively, to intersect the interval of maximum liquid petroleum availability. However, the amount of water in movement during the second stage, which does intersect this interval, is 10-15 per cent of the compacted bulk volume, and represents a significant fluid displacement capable of redistributing any mobile subsurface component. A measure of the degree to which this second-stage interlayer water has been discharged into the system can be noted on X-ray diffractograms. It appears to occur in a relatively restricted, depth-dependent temperature zone in which the average dehydration temperature of the points measured is 221 #176;F. With the use of an empirically derived P/T curve and a geothermal gradient map, a set of regional subsurface dehydration contours can be constructed. A plot of 5,368 liquid petroleum production depths referenced to this dehydration "surface" shows an almost perfect Gaussian distribution. It seems significant that, although the dehydration depths range from 4,000 to 16,000 feet, hydrocarbon production depths are distributed in a statistically consistent relationship to the calculated clay-dehydration contour surface.

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