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Upper Cretaceous sandstones of northern Mexico for a distance of 200 km (125 mi) south of the Rio Grande have similar mineral composition but differ markedly in reservoir quality in the north versus the south depending on the post-depositional history of the basins in which the sandstones were deposited. The sandstones are composed largely of detritus from volcanic and intrusive igneous rocks and were deposited in paralic and fluvial environments.
Sandstones in the north were never buried more than 1,000 m (3,300 ft) and were subjected to slow, gentle, basinward down-warping. They underwent a complex diagenetic history of cementation by chlorite, quartz, calcite, and kaolinite, and the development of modest secondary porosity; and they form hydrocarbon reservoir rocks of moderate quality (^phgr = 5 to 15%, k = 5 to 300 md). Sandstones to the south were buried rapidly by 1,000 to 4,00 m (3,300 to 13,000 ft) of younger strata and immediately thereafter underwent strong compressional folding and local thrust faulting during the Laramide orogeny. The sandstones lost from 20 to 35% porosity by compaction and the remainder of the porosity by cementation with calcite. They are tight, did not develop secondary porosity, and have no sho s of hydrocarbons.
During slow subsidence of sandstone-shale sequences in the north, the shale and associated organic matter underwent normal maturation events. Shale water was expelled in stages, organic matter evolved to produce liquid and gaseous hydrocarbons, and acid formation water was generated. The associated sandstones underwent a diagenetic sequence, including the development of secondary porosity from the acid formation water, that is typical of many sedimentary basins. To the south, Laramide compressive forces caused strong compaction and premature de-watering of the Upper Cretaceous shales. Most water present in the shales was expelled prior to maturation of hydrocarbons. Thus, compaction was rapid and severe and there was no opportunity for the development of typical formation waters that ight have developed secondary porosity. Close to the Sierra Madre front, rapid and early expulsion of water produced a strong fracture cleavage in shale and siltstone.
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