Part 1: Evolution and Deposition
This will be the first of a three part series where I will discuss the Permian Basin as well as the similarities and differences in the Midland Basin and the Delaware Basin. This first discussion will cover the evolution and deposition while the following will cover stratigraphy, reservoir quality, and production of this basin.
The Greater Permian Basin (GPB) is one of the largest and most structurally complex regions in North America. This sedimentary basin is comprised of several sub-basins and platforms. It covers an area about 250 miles wide and 300 miles long in 52 counties in west Texas and southeast New Mexico. That’s more than 75,000 square miles! Though it contains one of the world’s thickest deposits of Permian aged rocks, it was actually named after the period of geologic time (Permian: 299 million to 251 million years ago) where the basin reached its maximum depth of 29,000 feet.
The evolution of the basin can be attributed to three distinct phases: (1) mass deposition (2) continental collision (3) basin filling. Before the Permian Basin was formed, this region was a broad marine area called the Tobosa Basin. During the Cambrian to Mississippian periods (541 to 323 million years ago), massive amounts of clastic sediments were deposited in this area causing it to form a depression. What we define as the basin today began forming in late Mississippian and early Pennsylvanian (323 to 299 million years ago) when the supercontinents Laurasia and Gondwana collided to form Pangea causing faulting and uplift. While the area was covered by a seaway (figure 1), episodes of faulting, uplift, and erosion (associated with the Marathon-Ouachita Orogeny) as well as different rates of subsidence caused structural deformations in the larger Tobosa Basin that divided it into sub-basins and platforms.
The final process that created the GPB was the filling of the sub-basins with sediments. The Midland Basin, Central Basin Platform, and the Delaware basin are the three main components of the GPB that we know today. Other sections of the GPB include: the Northwest Shelf, Marfa Bain, Ozona Arch, Hovey Channel, Val Verde Basin, and Eastern Shelf.
The Midland and Delaware sub-basins share mutual characteristics such as age and lithology, but depths, nomenclature, and development vary throughout the GPB. The sub-basins rapidly subsided, while the platform remained at a higher elevation. This resulted in areas having very different water depths and depositional environments. The basins accumulated terrigenous clastics that are associated with deep water environments, whereas coarse grains associated with shallow reef environments were deposited along the platform. Differences in sedimentary depositions and tectonics initiated stratigraphic discontinuities between the two sub-basins.
The Midland Basin
The eastern Midland Basin accumulated large amounts of clastic sediments from the Ouachita orogenic belt during the Pennsylvanian (323 to 299 million years ago). As these sediments were deposited, they formed a thick subaqueous deltaic system that consumed the basin from east to west. During the Permian period, the delta system was covered with floodplains and was nearly filled by the Middle Permian.
The Delaware Basin
The western area of the GPB, the Delaware Basin, was a structural and topographical low that provided an inlet for marine water during most of the Permian. Minor sedimentation was received from the low coastal plains that surrounded the basin. While the Midland Basin was almost full of sediment by the Middle Permian, the Delaware became host to reefs built by sponges, algae, and microbial organisms. These organisms, along with the deep water inputs supplied by the Hovey Channel (figure 3), promoted carbonate buildups that formed a higher elevation area which separated the shallow water and deep water deposits.
Figure 3: Permian Map: The Hovey Channel supplied the Delaware Basin with deep water sediment, while the Midland Basin was restricted by carbonate reefs of the Central Platform, source: http://www.vyey.com/assets/permian-basin
Depth also had an important impact on the way sediments were deposited in the basin. The Delaware Basin is approximately 2,000 feet deeper than the Midland Basin (figure 4), thus causing the sediments to experience nearly twice as much pressure during burial. This is a leading factor in the stratigraphic discontinuities between the two sub-basins.
Figure 4: Depth map of the Delaware Basin, Central Platform, and Midland Basin, source: http://www.searchanddiscovery.com/pdfz/documents/2012/10412fairhurst/ndx_fairhurst.pdf.html
Still To Come…
Hopefully you now have a better understand of how the Greater Permian Basin, as a whole, evolved into the structure that we know it to be today as well as the relationships between the Midland and Delaware sub-basins. I will next discuss the stratigraphic differences between the two sub-basins and will later discuss how this is all related to the hydrocarbon production of these two popular plays. Stay tuned…
What do you think? Leave a comment below.
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