Abstract

An Apache Corporation 2016 presentation displayed control of 307,000 contiguous net acres, or 352,000 gross acres, along the Alpine High (AH). The eastern edge of their acreage play was bounded by an incipient subsidence fault down to the east trending NW-SE. Other faults on the AH trend parallel to this main fault, with some smaller faults trending NE-SW. The Grisham Fault (GF) zone extends east from the boundary fault.

The 1998 West Texas Earth Resources Institute (WTERI) tectonic map shows a more complex structural assessment. The GF zone subdivides the Delaware Basin (DB) into northern and southern domains. The AH, or Diablo Platform (DP) region, is sliced by numerous faults that form local highs and lows. The DP also has surface faults suggesting a long-lived structural history. The platform projects south into the Marathon uplift where the eastern platform boundary is defined by surface folding (PB King) and subsurface faulting defining uplifted western and depressed eastern domains (Muehlberger and Tauvers). An ancient DP thus underlies the western domain prior to Marathon deformation.

The GF defines the northern boundary of the Grisham Uplifted Region (GUR) extending from the AH across Reeves County into the westernmost part of Ward County. The GUR is bounded to the south by NE-trending interpreted dextral strike-slip faults, and to the north by the sinistral GF zone and subsidiary splay faults. The western part of the GUR is structurally higher as displayed on Woodford structure mapping. Maximum vertical offset across the GF is around 600 ms (>=3,000 feet), decreasing eastward to 250 ms. The difference in vertical offset is accommodated by reverse faults and folds across the western GUR.

While seismic and well data support vertical uplift, interpretation of GF sinistral strike-slip offset is based on comparing fault orientation and producing anticlinal structures with shear box experiments of strike-slip/wrench fault systems. Producing fields formed early in development of GF strike-slip offset as anticlines that were later ruptured and offset. This interpretation suggests deep folding is present off the north side of the GF zone, largely undrilled and awaiting development. Eventually vertical movement commenced along the GF with the south side uplifted several hundreds to thousands of feet.

The GF acquires additional importance when compared to Trans-Pecos E-W-trending fault structures identified using gravity and magnetics (Dickerson). The South Hueco structural zone extends east from the Trans-Pecos into the DB to become the GF zone. At least three other zones in the Trans-Pecos project eastward into the DB along fault zones mapped from seismic and well data.

The DB tectonic history is more complex than previously suggested in the late 1950’s, and carried forward into the 21^st Century. Most previous tectonic and structural descriptions are based on early work predating plate tectonic understanding. WTERI investigations conducted for an industry consortium suggested 8 tectonic and/or structural events affecting the stratigraphic history of the basin. Complex structural mapping of the DB and the connection to older reactivated structures in the Trans-Pecos would seem to support this general thesis.