In the Central Basin platform of the West Texas and south-eastern New Mexico larger Permian lays the system of Simpson Ellenberger petroleum. Initially, the system contained ready oil of 3 billion bbl. The petroleum system extends roughly 9600 sqkms. comprising in excess of 50 fields. In this span of area, sedimentary rocks range from tertiary to early Ordovician. Further, this region predominantly consists of Paleozoic strata, and roughly half of its breadth is Permian. Geochemical data consisting of gas chromatographic and isotopic composition indicate presence of Ordovician source rock. Besides, the geochemical data states that the oil-rich Ordovicia source rock was possibly confined to Shale gas possessed by the Simpson Group. Thermal constructions and urial preparation pint out that the oil was produced and thrown cover an area of roughly 210-m.y. period (Katz, Robinson, Dawson, Elrod, 1994).
The bulk of the basin rock of this petroleum system is dated within early Ordovician (karstified Ellenburger dolomites). The hydrocarbon collection that formed during the Ochoan period (late Permian) is typically faulted anticlines. The source rock, the primary basin and the sea rock accumulated over a 52-m.y. period. The primary overload rock for the migration of hydrocarbon and generation happened in a 418-m.y. period and culminated during the Oliggocene (~35Ma). Roughly, 180-540 billion bbl of the oil generated by the Simpson Group mentions that trapping efficiency of this system is nearly 4.3-14.3% in case a comparison to the initial estimate of oil institute (Katz, Robison, Dawson, Elrod, 1994).
The general concept of oil initiating in the deep, caving in basins and travelling into adjacent shelf-margin faces available in the Permian Basin is known. Vertical and lateral migration i.e. secondary migration of hydrocarbons in the Persian basin happened over vast stretches. The hydrocarbons that emanated from source rocks of greater depths which are more thermally mature moved into greater shallower and undeveloped reservoirs in the adjacent regions alongside the shelf margins. The available situation is a persistent ruptured network which helped the vertical communication among the reservoirs and the source rocks. In several situations, oil has flown into reservoirs downwards from the source rocks which are overlying (primary migration). For example the compaction of the Simpson Shale during the period of Middle Ordovician period lead to oil buildup in the Simpson Group's sandstones and in the underlying Ellenburger Dolomite as well. With the swelling of the burial broadening, the pace of the generation of hydrocarbon and resultant migration into the underlying Ellenburger gathered momentum. Several arrangements of the Paleozoic portion of the Permian Basin manifest the features of a reservoir. Established on the rising quantities of hydrocarbons generated, "Lower Ordovician, Middle to Upper Pennsylvanian, and Lower Permian structure contain the important reservoirs". http://www.britannica.com/EBchecked/topic/531752/secondary-migration
The Guadeloupian sequence, especially the San Andres and structures of Grayburg contain primarily prolific oil reservoirs and account for more than 50% of the oil discoveries in the Permian basin. Related to the Permian Basin reservoirs are structural, stratigraphic and combination (stratigraphic-structural) nature of trap. The major portion of the shelf-margin reservoirs adjacent to the basin is mainly connected with stratigraphic traps with several structural portions and pure stratigraphic traps. "As shelf margin carbonate systems aggraded and then protruded due to decreasing accommodation, facies tracts shifted basin ward resulting in sub tidal and intertidal carbonates overlain by intertidal carbonates, supratidal evaporates or siliciclastics" (Richard ,1998).
In majority of situations, the siliciclastics or the evaporitic facies outcome in barriers or seals to vertical migration of fluid, thereby forming a stratigraphic trap. Unequal compaction of sediment and reactivation of "preexisting faults deeper in the section cause compartmentalization of reservoirs" (Katz, Robinson, Dawson, Elrod, 1994). The digenesis of Meteoric which happens for the period of base-level fall and exposure of shelf might absorb anhydrite cementation in the depositional cycles and across the fault planes. In several situations, laterally extensive anhydrite gaps function as reservoirs seals.
Secondary migration (2010). In Encyclopædia Britannica. Retrieved from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/531752/secondary-migration
B. J. Katz, V. D. Robison, W. C. Dawson, L. W. Elrod (1994). Simpson—Ellenburger: Petroleum System of the Central Basin Platform, West Texas, and U.S.A.: Chapter 28: Part V. Case Studies--Western Hemisphere.
Richard C. Selley. (1998). Elements of petroleum geology. San Diego: Academic Press.