--> Abstract: Diagenesis and the Convective Flow of Formation Waters: An Example from the Norphlet Formation (Jurassic), Mobile Bay, Offshore Alabama, by R. W. Mitchell; #90987 (1993).

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MITCHELL, RAY W., Conoco Exploration Research and Services, Ponca City, OK

ABSTRACT: Diagenesis and the Convective Flow of Formation Waters: An Example from the Norphlet Formation (Jurassic), Mobile Bay, Offshore Alabama

The Norphlet Formation is characterized by a quartz-cemented cap and a lower porous zone. The Norphlet occurs between the impermeable Louann Salt and shaley limestones of the Smackover, so only an intraformational source of silica is likely. The distribution of cements and porosity match theoretical predictions of diagenesis resulting from thermal confection of formation water. Applying the convection model, formation waters along the lower contact were heated due to proximity to the underlying heat-conductive Louann Salt. The increase in temperature raised the solubility of silicate minerals in the formation waters in the lower Norphlet and dissolved the most soluble phases, i.e. microcrystalline quartz, calcic plagioclase, and silicate rock fragments. Convective cells formed due to luid density instability. The relatively homogeneous nature of the Norphlet eolian sandstone and lack of internal barriers allowed the warmed waters, saturated with respect to quartz, to rise until reaching the contact of the sandstone with the overlying Smackover. There the fluid cooled, lowering the solubility of quartz and triggering the precipitation of quartz cements as overgrowths on detrital grains. Upon cooling, the density of the water increased and fluids flowed down through the formation to be reheated and recharged with silica near the lower contact.

The thickness of the tight zone varies dramatically from well-to-well, possibly due to the location on anticlinal/domal structures; the model predicts that the tight zone would be thickest on the crests and thin down the flanks of structures. In addition, the size of the structure and the development of early fault patterns may affect the development of the tight zone. Recognition of convection as an important fluid flow mechanism may allow some understanding of the irregular distribution of the tight zone of the Norphlet.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.