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Investigation of structure, temperature, pressure, salinity, and core samples at Kaplan field yields information on diagenesis of fine-grained sandstones deposited in an outer shelf/upper slope depositional environment. Cross sections and structural maps reveal a domal structure at 15,000 ft (4,572 m) of depth and a northeast-striking growth fault. Post depositional faults occur at shallower depths (11,500 ft; 3,505 m). A large growth fault forms the northern border of the study area. The shallow occurrence of
geopressure is related to structure and a high shale/sand ratio. Low isothermal surfaces in the down fault blocks accompanied by anomalous high temperatures in the upthrown blocks indicate vertical leakage of fluids along growth faults from underlying geopressured aquifers. The association of low salinity fluids (less than 60,000 ppm) with leakage zones affirms structural control of fluid movement through the Anahuac and Frio formations (Oligocene) at Kaplan field.
The Frio Formation core samples from 16,700 to 19,600 ft (5,090 to 5,974 m) of depth, representing channel and channel-edge turbidite sandstones, were examined petrographically and by SEM. The arkosic composition of late stage diagenesis sandstones at Kaplan field suggests an original arkose or lithic arkose composition (classification of McBride). Nonferroan calcite cementation, chlorite rims and cement, and quartz overgrowths characterize early diagenesis. At a middle stage of diagenesis secondary porosity is developed by dissolution of unstable grains and calcite cement. Samples flushed by geopressured waters from greater depth show kaolinite pore-fill and quartz overgrowths, chlorite (polytype IIb) and illite cement, and feldspar overgrowths in the late diagenetic stage. Premetamo phic textures are apparent in the deepest section at 338°F (170°C).
The low permeability of sandstones with extensive early chlorite cement (channel-edge sandstones) precludes development of extensive secondary porosity. In contrast, sandstones with little early chlorite cement develop and maintain secondary porosity through the late diagenetic stage. Restriction of fluid movement by early chlorite cement has ramifications for migration of hydrocarbons or geothermal waters, and for gas production at Kaplan field.
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