Facies Control of Cementation and Porosity, Pennsylvanian Fan-Delta Sandstones, Texas Panhandle: ABSTRACT

Abstract

Interbedded fan-delta sandstones and limestones were deposited on a shallow carbonate shelf in the southern Anadarko basin during Missourian time. Hydrocarbon production from the fan-delta sandstones at Mobeetie field, Wheeler County, is controlled both by structure and facies-determined porosity distribution. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. Diagenetic history of the distal, marine-reworked sandstones was strikingly different from that of the more proximal, non-reworked sandstones. The first cement to precipitate in the reworked sandstones was a thin, isopachous rim of Mg-calcite cement that probably precipitated in the submarine environment soon after deposition. Next, establishment of a freshwater, phreatic environment in the sediments resulted in extensive calcite cementation in the calcareous sandstones. Dissolution of aragonitic oolites and fossils provided the source of the calcite that occluded primary porosity. In contrast, the non-reworked sandstones were not cemented because they lacked a calcite source, and so they retained high porosity. Rims of authigenic chlorite, which reduced porosity by only a few percent, were the earliest cements to precipitate in the non-reworked facies. With increasing burial, porosity in both the reworked and non-reworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. These cements are generally minor in volume and do not influence porosity distribution. Generation of secondary porosity by dissolution of feldspars and rock fragments occurred in all sandstones but was more extensive in non-reworked facies. However, the main control of present porosity distribution is the presence or absence of early, freshwater calcite cement. End_of_Article - Last_Page 565------------