ABSTRACT
Pennsylvanian arkosic sandstones at Mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern Anadarko basin. The detrital minerals of the sandstones reflect the composition of the Precambrian granites and granodiorites that were exposed in the nearby Amarillo uplift. Distal margins of some fandelta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous elastics. The diagenetic history and, hence, reservoir quality of the distal, marine-reworked sandstones differ from those of the more proximal, nonre-worked sandstones.
The earliest diagenetic events—formation of chlorite ooids and precipitation of fibrous submarine Mg-calcite cement—took place in the depositional environment of the marine-reworked sandstones. On the basis of isotopic data, the remainder of the diagenesis evidently occurred in fluids of meteoric origin. Shallow, early meteoric diagenesis induced precipitation of iron-poor calcite spar in reworked sandstones; nonreworked sandstones lack calcite spar but contain early pore-lining chlorite cement. Dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones.
As burial increased, porosity in both the reworked and nonreworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. Oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition.
The last diagenetic events were influenced by the overlying Permian evaporites. Sodium-rich fluids caused partial albitization of detrital plagioclase, and sulfate derived from the evaporites precipitated as anhydrite and celestite.
