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Deep exploratory walls in the Scotian basin, offshore Nova Scotia, have found overpressured formations with high pressure gradients. The overpressures occur over an area of approximately 10,000 km2 centred around Sable Island and the Venture Gas Field. In the Venture area, the abnormal pressures are confined below a depth of 4500 m and are associated with gas- and condensate-bearing sandstone reservoirs in the lower Missisauga and Mic Mac Formations, of Lower Cretaceous-Upper Jurassic age. The overpressured zone is more than 1200 m thick and its "floor" has not been reached yet. Virgin reservoir pressures range from nearly hydrostatic to 0.89 times of geostatic gradient with increasing depth.

Venture overpressures are atypical and differ from Gulf Coast type overpressures in that they occur within well compacted strata containing numerous sandstone reservoir beds. The overpressured Venture strata comprise a varied rock package dominated by well indurated sandstones, shales, and limestones. Normal compaction in these strata is indicated indirectly by geophysical logs which show trends of increasing bulk density, sonic velocity, and shale resistivity with depth. Plots of temperature gradients, organic maturation gradients, drilling rates, and chemical composition of formation waters versus depth across the overpressured zone further indicate the normal compaction of the rock sequence. Detailed XRD and SEM analysis shows that overpressured Venture shales are well indurated and compacted and do not differ in texture or clay mineral content from the overlying, normally pressured shales. Petrographic studies show that the sandstones are also severely compacted and cemented, while available porosity is secondary in origin. Deep Venture overpressures are not associated with undercompacted sediments and hence they cannot be predicated from compaction trends.

Mic Mac-lower Missisauga sandstone reservoirs were deposited as sand ridge/bar complexes, shoreline/coastal bar complexes, and oolitic shoal complexes is a prograding middle-inner shelf to nearshore-delta front environment. The sandstones are predominantly fine-to medium-graded, well to moderately-sorted quartz arenites. Their depositional porosity may have been as high as 40 percent. The sandstones underwent stages of cementation, leaching, and recementation with burial depth. Early cementation occurred under normal pressure conditions, leaching and recementation under transitional to overpressured conditions. During burial under normal fluid pressure conditions the sands were mechanically compacted and were cemented by iron-poor sparry calcite, early-quartz/feldspar overgrowths, chlorite rims, and pore-filling ferroan dolomite (Eodiagenesis to Semi-Mature Mesodiagenesis). Precipitation of these early cements probably reduced primary porosity to less than 5 percent and permeability to several millidarcys or less. At greater depths of burial and under transitional to

overpressured conditions, pore-filling dolomite was dissolved and secondary porosity created (Early-Mature Mesodiagenesis). Dolomite leaching was accompanied by minor framework grain repacking and by the precipitation of small amounts of late syntaxial quartz and kaolinite cements. Overpressuring of the rock sequence at this stage, resulting from hydrocarbon generation in the basin, prevented further compaction and pore collapse, and helped to preserve secondary porosities greater than 30 percent and permeabilities of several hundred millidarcys in deep Venture sandstone reservoirs. With deeper burial, minor amounts of late-ferroan dolomite were precipitated (Late-Mature Mesodiagenesis). Subsequent to this, Venture sandstones were profusely flushed with overpressured gases and condensates, bringing to an end the history of diagenetic cementation and pore space alteration in these sandstones.

The Venture overpressure system is interpreted to be a hardrock overpressuring occurring within normally compacted strata. The overpressures appear to be a "recent" phenomenon generated "late" in the burial history of the basin. The overpressured zone brackets the "hydrocarbon-gas window" and is vertically confined to the portion of the basin within which large amounts of gas, condensate, and other volatile hydrocarbons were generated, expelled, and trapped. Venture overpressures appear to have been caused by hydrocarbon generation processes deep in the basin.

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Supervisor: L. Jansa /