| dc.description.abstract | The “Mississippi Lime” has been a productive carbonate play for several decades. However, geologic controls on production and reservoir quality have frequently been ambiguous to geologists. The Mississippian limestone varies significantly in reservoir quality, with some zones characterized by higher porosity tripolite, other parts characterized by lower porosity chert, and some characterized by unaltered limestone.
Lithologically, the Mississippian limestone consists of tripolite, chert, cherty limes tone and limestone. The dominant lithofacies include tripolitic chert-breccia (tripolite), skeletal grainstone, skeletal-pelo idal packs tone, bioturbated wackestonepackstone, and bioturbated mudstone-wackestone. Tripolitic intervals and brecciation occur most often near the top of the Mississippian interval, and commonly represent the best reservoir in terms of porosity and permeability. The peloidal packstone facies has on average the second highest porosity of any facies, but is significantly lower than tripolite. This variability (both vertical and lateral) is established through cores and thin sections. Mississippian-aged limestones demonstrate a variety of alteration types , such as silicification, dolomitization, brecciation, and fracturing . Porosity is highly variable and most orten a function of alteration (mostly the amount of dissolution). Pore type is also variable, but is predominantly vuggy, while other types, like moldic and fracture porosity depend on facies type and degree of alteration.
In northern Oklahoma, the Mississippian limestone formed on the east-west
trending margin of the Anadarko shelf/ramp. This carbonate system is characterized by a shallowing-upward character as well as high-order transgressive-regressive cyclicity. Five shallowing-upward fourth-order cycles are observed in the Mississippian in the study area. based on cores and thin sections. Well-log response also shows a degree or cyclicity, as well as the progradational nature or the carbonate ramp. Unconformities in the area are caused by relative falls in sea level in addition to regional tectonics. PrePennsylvanian tectonics created the Nemaha uplift, the cause of subaeria l exposure in the area, which then led to alteration and brecciation or the rocks.
Three-dimens ional reservo ir models constrained to well-log data from the show five fourth-order cycles that thicken as they prograde to the south down the carbonate ramp. Porosity, resistivity, and bulk density petrophysical models show favorab le reservoir zones. These zones are irregularly distributed, but are highly concentrated in the regressive phase of the Mississippian third-order seque nce. Pcloidal packstone intervals, which have slightly bener reservoir quality than the other facies, could be predicted based on the stratigraphy and cyclicity, but are less porous and permeable than tripolitic-chert intervals. Tripolitic reservoirs arc controlled predominantly by the amount and distribut ion of alteration. Stratigraphically lower cycles show less lteration, and therefore lower reservoir quality. The degree and areas of diagenetic alteration as well as the sequence- stratigraphic framework provide the main controls on reservoir quality and its distribution. | |
