A little scientific but some good explanations of causes for...

A little scientific but some good explanations of causes for overpressured reservoirs. The shale diapirs in the area could have brought gas filled, porous reservoirs from lower and higher pressured depths and the surrounding shales trapped the pressure. I am optimistic.


Over pressured formations are much more common than subpressures and are encountered world wide in sediments varying in age from Pleistocene to Cambrian. Whereas normally pressured formations are considered "open systems" permitting hydraulic communication of interstitial fluids with the surface; abnormally pressured formations of interest are usually found to be "closed systems" which have been geologically pressured. In this case by forcing the formation to maintain it's fluid content and in doing so, cause it to become abnormally pressured, the permeability barrier acts as a pressure seal. In a geo-pressured sequence of shales and sands the shales composed primarily of 'platy clay minerals" fill the role of the permeability barrier. In such a sequence the ratio of shale to sand must be fairly high in order to increase the possibility of a sand unit being completely isolated and encapsulated by the surrounding shales. The creation of an over pressured formation is related to many physical, geochemical and mechanical processes and their order. "Genesis" of over pressure during geologic time is controlled by the environment of deposition on the paleo-continental shelf, and slope, the geometry and lithology of the sediments, regional and local faulting, basin hinge lines, burial and compaction, and subsequent structural deformation.

Generally for a formation to be producing reservoir it must be porous. Potential reservoirs are either geologically or hydrologically pressured. Of those that are geo pressured systems the pressuring of the reservoir may be either primary (originates at reservoir) or secondary (transmitted to reservoir from a deeper overpressured zone). Factors which affect the formation of overpressured zones include: (1) the natural occurrence of reservoir structures, (2) the rate of deposition of sediments and the depositional environment, (3) the amount of uplift and erosion, (4) the tectonic activity in the area and (5) diagenetic processes.

Other minor factors which are thought to contribute to the formation of an over pressured zone are, osmotic phenomenom, massive salt bed deposition, and permafrost encroachment.

For a hydro pressured system, these conditions can occur only at relatively shallow depth, where the degree of compaction is not such as will cause the formation to nonporous. In this case the porous and permeable aquifers are structurally situated between two impermeable beds and the aquifer is structurally deformed so that the necessary hydraulic "head" may be generated to pressure the formation.

The natural structure of a reservoir may lead to pressuring of the capped area. Water pressures which may be normal at the base of the zone are transmitted to over lying oil and gas pockets thereby creating an overpressured gas formation at the top of the layer. Most of normally pressured formations, however, are located within shale sequences who's initial sediments were deposited at a fast rate causing a premature development of a permeability barrier which restricts fluid expulsion before compaction is complete. (Factors other than the rate of sedimentation which effect formation of an overpressured zone are: (a) total thickness of sediments, (b) presence of clay rocks, (c) shale to sand ratio in interbedded sequenced, (d) slope of sedimentary basin.) In this situation the water will, instead of moving vertically, be squeezed into the adjacent sand sequences creating an overpressured sand formation. Shales and sands in this type of sequence will illustrate a high porosity and low bulk density.

Enclosed porous rocks which have been stabilized with respect to pressure at great depths and then through the processes of up lift and erosion being transported to shallower depths will show evidence of abnormally high pore pressures.

Tectonic activity within the area of interest may contribute to secondary charging of a reservoir. Through tectonic action such as local and regional folding, faulting, sliding and slipping, earthquakes and diapiric shale and salt movements, it is possible that the vertical geologic sequence in an area may be rearranged and an impermeable barrier situated in an appropriate position. It is also possible that a fault plane may cause a pressure link between an overpressured formation and an upper normally pressured formation. Shale and salt diapirs are formed through the upward migration of substances of relatively low density through layers of material of relatively higher density. Massive salt and shale deposits gradually accumulate and move upward penetrating over lying sediments, creating impermeable barriers and causing associated faults. Less commonly, overpressured formations may result from diagenitic processes. This post depositional alteration of sediments and the constituent minerals may result in the formation of minerals which occupy a greater amount of space which increases the volume occupied by the rock matrix. For example, the hydration of anhydrite to gypsum increases the bulk volume of the mineral by 40%. Occasionally thin shale sequences respond as semipermeable membranes in the presence of excessive salt solutions, contributing to the development of an osmotic pressure differential between the two formations on either side of the shale. Osmotic pressures are most commonly associated with evaporate deposits and have been known to create pressures sufficient to rupture the reservoir.

1.14 Pressure Detection Theory

Special structural characteristics or series of traits are unique to formation which contain high pore pressures. These characteristics may include evidence of a pressure barrier of fault plane associated with the abnormally charged formation. It is this evidence which must be detected to enable an operator to be prepared for the pressures to be encountered. Various operations performed both before and during the drilling operation are utilized to aid in location of possible over pressured zones.

Prior to spudding the well, the operator will use geophysical techniques, such as sonic logs, to search for any possible pressure anomalies as well as to indicate the location of possible formations tops. During the drilling of the well both drilling and drilling fluid parameters are observed and the rock cuttings circulated up from the bottom of the hole are carefully examined. These observations which are a particular interest to the mud logger, include drilling rates, mud weight, flow line temperature, torque and bulk shell density. With an understanding of the interrelationships between these various factors and normally and over pressured zones, trained personnel may interpret the data to optimize drilling programs and detect abnormal pore pressures while drilling. Other techniques which involve the usage of direct pressure measuring devices and wire line tools may be utilized upon completion of the drilling of the well to obtain more information in reference to potential production zones.

As an overpressured formation is approached marked differences in degree of compactions, porosity and minerals composition of interstitial fluids occurs. Also the formation pressure may begin to rise and approach the bottom hole pressure thus decreasing the bottom hole pressure differential. If properties which are affected by such factors are closely monitored and plotted with respect to depth, an abnormally pressured zone may be suspected when a distinct deviation from an average line trending with depth is observed.

It must be remembered though, that such trends may also vary slightly with change in the lithologic composition of the unit and will be interrupted totally by a formation change. Such minor deviations must be recognized and interpreted as such to distinguish them from more significant anomalies associated with a possibly abnormally pressured formation.

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