Facies-Dependent Diagenesis of Shelf-Margin Sandstones and Siltstones, Yates Formation, Northern Margin of Delaware Basin

J. M. Borer, P. M. Harris

Shelf-margin sandstones and siltstones of the Permian (late Guadalupian) Yates Formation, continuously cored in the Gulf PDB-04 research well of the northern margin of the Delaware basin, have varying textures, compositions, and diagenetic histories. Gray, dolomitic, quartzose, very fine to fine-grained sandstones occur in the lower part of the formation; dark red (FeO₂-stained detrital clay-bearing), anhydritic, subarkosic to arkosic, very fine-grained sandstones/siltstones occur in the upper portion. Stratigraphic relations, associated carbonates, and ichnofossils suggest that the gray sands were deposited seaward of a pisolite shoal complex in an intertidal to marginally subtidal setting, whereas the overlying red clastics accumulated along the crest of the hoal in a supratidal environment. This interpretation is congruent with the progradational history of the Yates Formation as established by outcrop and subsurface studies. Depositionally updip sandstones, studied from outcrop and other wells, show that the source sandstones for the gray and red sandstones in the PDB-04 well have relatively uniform depositional textures and mineralogies (silty, fine grained, lithic, subarkoses to arkoses). This strongly suggests that the variations seen in the core are due to facies-dependent diagenesis.

Stratigraphic and petrographic data indicate that both syndepositional-shallow burial and telogenetic(?) diagenesis were controlled by depositional facies. The presence of winnowing currents during deposition along the shelf edge produced better sorting and slightly coarser grain size in the gray sandstones, allowing for their increased permeability. The gray sandstones were cemented by minor amounts of authigenic pyrite and quartz overgrowths. Dolomitization [including cementation, recrystallization of original micrite, and replacement of clastic detritus(?)] largely occluded primary porosity. Later diagenesis resulted in nearly total dissolution of feldspars, growth of associated kaolinite, and precipitation of coarse dolomite and calcite cement. Secondary dissolution porosity is mi or (3-6%), whereas microporosity associated with kaolinite is locally abundant. Porosity development is interpreted as being related to the proximity of the shoal-front depositional setting to porous reef, immediate back reef, and shoal-front carbonates that experienced meteoric flushing in the Pleistocene following uplift of the northern margin of the basin.

Diagenetic phases in the red sandstones/siltstones include minor grain-lining illitic clay [recrystallized micaceous detrital clay(?)] and abundant early precompactional anhydrite, magnesite, and dolomite cements filling intergranular porosity. Abundant preserved feldspar, a paucity of kaolinite, and little porosity in these sands are attributed to relatively early evaporite and carbonate cementation in the shoal-crest position. This early cementation, along with the finer mean grain size (more detrital clay and silt) and greater distance from the porous reef carbonates, precluded later diagenesis.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.