a#1 took a gas-kick, page-33

hope this helps -

Bob

"1.13 Occurrence and Origin of Over-Pressured Formation
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
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