Facies, cycle stratigraphy, and heterogeneity of the Clear Fork carbonates in the Eastern Shelf of the Permian Basin, Texas

Abstract

The Clear Fork carbonate reservoirs at Iatan East Howard field are highly heterogeneous. Facies analysis and cycle stratigraphy are employed to characterize the Clear Fork Formation and reservoir heterogeneity at the field. One-dimensional (1-D), vertical stratigraphic analyses started from meter-scale cycles to cycle sets, system tracts, and then depositional sequences. A reverse process was followed to construct a 2-D framework from a 1-D stratigraphy. In the downdip areas, the Clear Fork Formation is dominated by subtidal facies; toward the updip, peritidal facies become more abundant and depositional facies may alternate between shallow subtidal and tidal-flat facies. Approximately 56 well-developed, meter-scale, upward-shallowing cycles were defined in the Clear Fork cores facilitated by wireline logs. Most of them have bioclast wackestone and packstone in the lower part and are capped by fenestral peloid packstone, silty to sandy dolostone, and/or dark mudrocks. Six cycle sets (high-frequency sequences) were identified, and two depositional sequences were recognized. Sequences are correlatable, but it is challenging to correlate meter-scale cycles. The Clear Fork cycles display relatively consistent thickness and facies stacking pattern plus evidence of subaerial exposure and erosion of subtidal facies, suggesting an allocyclic control and having been the result of eustatic sea-level fluctuations. Reservoir pores (porosity >4%) commonly occur in the transgressive system tracts and lower highstand system tracts (HSTs) within individual high-frequency sequences; the upper HSTs commonly consist of amalgamated peritidal deposits and are generally tight because pores (mostly fenestral and intergranular pores) have been occluded by anhydrite and carbonate cements. These new findings contrast the previous studies showing that porosity is typically higher in the cycle-set-top or cycle-top facies (mainly tidal-flat deposits) in the formation. Reservoirs are areally discontinuous and vertically compartmentalized by multiple nonporous zones. Reservoir characterization is critical to improving recovery efficiency. This case study provides an insight into complex facies and cycle stacking patterns as well as stratigraphic architecture on a carbonate platform during a waning icehouse phase and can help to better understand the controls on the distribution and quality of such carbonate reservoirs.