Abstract

The tectonic history of the Permian basin has controlled:

1. Distribution of shelfal and basinal source beds and their Organofacies;

2. Thermal history;

3. Juxtaposition of younger source beds on older carrier/reservoirs via unconformities, creating numerous charge access pathways;

4. Overpressure development, augmenting production in basinal unconventional reservoirs.

Based on regional analysis of remnant and restored isopachs, burial history, tectonic subsidence / thermal modeling, and pressure observations, we discuss the over-arching controls on Permian Basin petroleum systems. A 3-D restored burial history illustrates the timeline.

Pre-Cambrian Basement Terrains

Lateral variations in basement radiogenic heat production can govern local ‘hot sweet spots’ in the maturity, GOR and viscosity of unconventional reservoir fluids.

Early Paleozoic Passive Margin “Steer’s Head”

Paleozoic sedimentation commenced in the precursor Tobosa Basin in the late Cambrian. Above lower Ordovician carbonates there are three Paleozoic source intervals: the Simpson Group (mid-Ordovician), the upper Ordovician -Silurian, and the Woodford Formation (upper Devonian).

Thin-skinned compression and the Ouachita-Marathon foreland basin

Pennsylvanian convergence led to formation of foreland basins: southward-thickening in the Permian Basin, and eastward-thickening in the Fort Worth Basin. Erosion on the Concho/Texas Arch, and elsewhere, juxtaposed Pennsylvanian source rocks above carrier/reservoir systems as old as Cambrian.

In the Fort Worth Basin, initially rapid Atokan loading slowed by the mid Pennsylvanian; rebound began in the Virgilian. In the Val Verde Sub-Basin, foreland subsidence occurred later, post-Desmoinesian. By end Missourian, the depocenter shifted northward and turbidite channel sands were shedding off the eroding mountains along the southern margin into a now northward thickening basin.

Thick-skinned ‘Escape Tectonics’

By the Atokan, compression began to differentiate the CBP and Ozona Arch as thick-skinned transtensional / transpressional uplifts, with a steep faulted margin into the Delaware Sub-Basin and a gentle dip into the Midland Sub-Basin. Escape tectonics intensified beginning in the earliest Permian on the CBP, resulting in pre-Wolfcamp erosion on the crest and very thick Wolfcampian and younger basinal sediments in the Delaware Sub-Basin.

Thick Ochoan evaporites in the Delaware Sub-Basin contribute to the low geothermal profiles observed. Basin differentiation led to clay-rich Organofacies B deposition of Penn-Guadalupian age source rocks in the Midland and Delaware Sub-basins, with Organofacies A Wolfcampian to Guadalupian carbonate-hosted equivalents on the platforms.

End of Permian subsidence; mid Triassic erosion and late Triassic-early Jurassic Sag Basin By end Ochoan, bathymetry had been filled and subsidence due to escape tectonics ceased. Erosion removed from the basin margins the Ochoan and some older Permian section.

Pacific passive margin subsidence extended into the area depositing a modest section, of which only the Upper Triassic Dockum Formation is preserved.

Gulf of Mexico Steer’s Head and Sevier-Laramide Foreland

Pre-Mid Cretaceous uplift and erosion removed Triassic, and on the margins considerable Permo-Pennsylvanian, section. By earliest Cenomanian, transgression from the Gulf of Mexico met with Arctic Ocean waters, creating the Western Interior Seaway over this subcrop surface.

Accelerated loading from the Rockies created a westward thickening foreland wedge from Late Cretaceous-early Paleogene, which is preserved in the Raton Basin and the Big Bend area. This overburden accounts for the maturation state of Permian Basin source rocks and the preservation of over-pressures.

Mid Tertiary thermal plume; Basin and Range Extension

Rebound and erosion were coeval with 35 MY volcanics. Long wavelength extra-basinal tectonic uplift related to hot-spot subduction led was accompanied in the Miocene by Basin and Range extension, accounting for present elevations 3000’-4000’, reaching 9000’ on local footwalls. This erosion removed almost all of the Cretaceous-Paleogene and some of its sub-crop, down to the Wolfcampian. Local eastward tilting into the Delaware Sub-Basin complicated the relationship between maximum maturity and present burial depth.

Sediments <35MY played no part in the petroleum system: re-burial did not exceed Paleogene maximum burial.