Sequence Architecture and Climate Controls on Diagenesis Related to Subaerial Exposure in Pennsylvanian and Permian Cyclic Carbonates, Western Kansas

Govert J.A. Buijs and Robert H. Goldstein
University of Kansas, Lawrence, KS

Subaerial exposure of Pennsylvanian and Permian sequences from the western Hugoton embayment in Kansas resulted in predictable patterns of carbonate diagenesis with fabric-selective dissolution and fine, blocky calcite cement and overgrowths. The observations show how sequence stratigraphy and shelf setting can be used to predict porosity in carbonate reservoirs.

In the RB core, most early cements show distribution of growth zones indicating formation during multiple events of subaerial exposure. These cements precipitated in freshwater or in evaporated marine-meteoric mixing zones, as indicated by δ18O between ?5.7? and ?1.2? (VPDB), δ13C between ?2.5? and 0.7‰ (VPDB), and primary fluid inclusions with salinities between 5.3 and 0.0 wt% NaCl equivalent.

Amount of early calcite cement is controlled by sequence architecture and to a lesser extent by climate during subaerial exposure. Sequences formed during high sea level (S-architecture) contain more early calcite cement than sequences that reflect deposition well down on the falling leg of the sea-level curve (C-architecture). Duration of subaerial exposure was apparently longer for S-architecture than for C-architecture positions. At early stages of relative sea-level fall, climate was relatively humid, leading to more early calcite cement. At late stages of relative sea level fall, climate was relatively arid, leading to less early calcite cement.

The relative importance of mixing-zone cementation may be best explained by the setting of the RB core, where the low slope and interstratified siliciclastic aquitards decreased recharge and flow of meteoric groundwater, leading to very little flux of meteoric water and very little meteoric calcite cement.