Rhythmic Carbonate Versus Spiculite Deposition in Mississippian Hydrocarbon Reservoirs in the Midcontinent Usa: Causative Factors and Resulting Reservoir Petrophysical Attributes
Subsurface Osagean and Meramecian strata in the central midcontinent USA (southern Kansas and northern Oklahoma) generally comprise stacked, shallow-water, carbonate rock-dominated depositional sequences. Relatively thin sections of shallow-water spiculites are present locally in this section, however, and they are interspersed within the carbonates. In contrast, a considerably more prolonged period of spiculite deposition is represented by the Cowley Formation, which was deposited during the late Osagean to early Meramecian over a vast subsurface area that includes parts of Kansas, Oklahoma, and possibly panhandle Texas. Carbonate deposition was largely suppressed during this time, and instead, a thick (400-500 ft) sequence of shallow to deep-water spiculite and shale was deposited. Such deposits are not present in surface exposures in the midcontinent. Spiculites and associated carbonate rocks, particularly the thick spiculites in the Cowley, comprise prolific hydrocarbon reservoirs in Kansas and Oklahoma, and there are significant differences in reservoir petrophysical attributes and performance in these contrasting lithologies. Inasmuch as thick spiculites may be a major future play in the central midcontinent, knowledge of the factors that controlled carbonate versus spiculite deposition, and their subsequent diagenesis, are relevant to petroleum exploration and exploitation. Three main inter-related causative factors that appear to have effected rhythmic carbonate versus spiculite deposition are: (1) paleotopography of antecedent depositional surfaces, (2) paleo-latitudinal setting, and (3) oceanic circulation patterns and upwelling within the precursor Anadarko Basin. Possible controls on short-term versus longer-term periods of spiculite deposition, and on local versus regionally more widespread spiculite occurrence, remain enigmatic.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009