A knowledge of the geometry of the rocks in the increasingly deeper basin areas may provide significant geological elements which can affect drilling and completion procedures.
In the Mid-Continent "hard rock" country, porosity variations at depth do not follow the pattern that is recognized in the soft Gulf Coast sediments. Porosities in sandstones may vary with the presence of shale beds which are subject to compaction by overburden presence of shale beds which are subject to compaction by overburden pressures; and by factors of re-arrangement of grains accomplished pressures; and by factors of re-arrangement of grains accomplished through crushing and re-cementation. Grain size distribution and initial porosities will be factors to be evaluated. Fairly coarse, non-shaly, beach sands can be expected to porosity at depths, than deposited in deep water, are shaly. These latter poor porosity because of overburden pressures, as re-arrangement factors.
Limestone porosities averaging over 12% are being reported from Hunton (Devonian - Silurian age) limestone reservoirs below exhibit greater those sands which were below wave base, that types will exhibit compaction due to well as the 20,000 feet. Because of the rigid rock fabric, porosity in limestones is not expected to be a function of depth of burial. Rather, the environment of deposition will affect the primary porosity. The geologic history, of the record of periods of deposition, uplift, and mountain building movements in the area, will be the most significant factor in the development of secondary prorosity from which giant gas production is being proved.
The degree of dolomitization of limestones is another factor in the development of porosity. There is usually a straight line relationship between the increase in dolomite content of a limestone and its increasing porosity. Evidently dolomitization is an early phenomena, controlled by the depositional environment of the limestone and the diagenetic changes which are imposed upon the new rock. Any petroleum that is generated in a basin may move into the porous limestone reservoir rock from an adjacent shale source rock.
Increased fluid pressures at depth may initiate micro-movements along minute fracture planes, and thus add a factor to the drillability of a given rock, and may influence the migration of fluids.