WIXLIB

. 36TOBOSA BASINeroded surface with many karst features such as collapsebreccias, cave sequences, and associated cavernous anddissolution porosity (Canter et al., 1992; Fig. 12). Thewidespread occurrence of this exposure surface suggests thatit was related to a platform-wide drop in sea level. Paleokarstis found in the Fusselman throughout the Permian Basinsubsurface and contributes substantially to the reservoirporosity of producing fields (Mazzullo and Mazzullo, 1992).Wristen FormationBSILURIAN, DEVONIAN, ANO MISSISSIPPIANDEARLY PERMIANELATE PERMIANA subsurface interval previously referred to as the"Silurian shale," "Upper Silurian," or "Green Silurian" wasnamed the Wristen Formation by Hills and Hoenig (1979),the type section being described from a well in the Wickettfield, Ward County, Texas. Canter et al. (1992) reported thatthe Wristen Formation is thickest (450 m) in the northeasternpart of the Delaware Basin, but that it thins northward on theNorthwest Shelf and eastward on the Central Basin Platform.The Wristen Formation is primarily a mud- and shale-richrock dominated by dolomite.Following a post-Fusselman lowstand, a major transgression occurred during early Wristen time. First there was a risein sea level followed by a significant sea level fall later in theWristen, then there was another highstand in which thelatest, uppermost intervals of the Wristen were laid down(Canter et al., 1992). The sea level lowstand resulted inexposure and some karstification on a local scale. Thecontact between the Silurian Wristen and overlying LowerDevonian Thirtyone Formation is unconformable (Fig. 12).This unconformity is indicated by erosional truncation andfurther significant regional karst development.FIGURE 11—Conceptual diagram showing the development of theTobosa Basin into the Midland Basin, Central Basin Platform, and DevonianDelaware Basin from the mid-Ordovician to Late Permian. Crosssections across Lea and Eddy Counties, New Mexico. AfterThirtyone FormationCheeseman(1978).white to light-gray, coarse- to medium-crystalline dolomitewhich contrasts rather sharply with the darker-colored andfiner-grained Montoya (Hayes, 1964). Fossils found in theFusselman include Pentamerus, Amplexus, and Favosites. Inthe southeastern, Sierra Madera-Glass Mountains part of thebasin, the Fusselman has been reported to vary between 1550 m in thickness (Wilshire et al., 1972; Bloom, 1988). Inthe Hovey area, the Fusselman is 20 m thick in Dodson'sTexas American Syndicate no. 1 (Stevens, 1957).Regional emergence in the Late Silurian through MiddleDevonian ( 40 my duration) was accompanied by widespreadkarstification (Mazzullo, 1992). Several distinct karst eventsthat affected Silurian/Devonian strata are: (1) top of theFusselman, (2) top of the Wristen, (3) top of the Thirtyone,and (4) pre-Woodford. The contact between the Fusselmanand overlying Wristen Formation represents a majorunconformity and is typically described as an irregular orAt the end of the Silurian and into the early Devonian,shelf carbonates were deposited along the shallower marginsof the Tobosa Basin and dense limestones, cherts, and blackshales accumulated in deeper waters (Fig. 11B). Carbonatedeposits reached over 300 m in thickness during this time(Netherland et al., 1974). The Thirtyone Formation was oneof the carbonate deposits laid down in the Tobosa Basinduring the Early and Middle Devonian (Fig. 8).The name"Thirtyone Formation" was proposed by Hills and Hoenig(1979) for a unit previously known as the "Lower Devoniancherty limestone" or the "Devonian limestone-siliceous." Thetype section was described from a well in Blk. 31, CraneCounty, Texas, where the formation consists of about 300 mof light-colored siliceous and cherty limestone, with an upperinterval composed mostly of crinoid-rich limestone andminor sandy limestone. The "Devonian" in Reeves, southwestern Pecos, and northern Brewster Counties has beendescribed as being almost entirely chert and having athickness of 32-43 m (Bloom, 1988). The "Devonian" is 80 m

38The top and base of the Mississippian limestone is usuallyeasily recognizable in well samples and mechanical logs(Grant and Foster, 1989).The Mississippian in Kalinga Corporation's Margaret no.1 well, along the Hovey anticline in the Glass Mountainsarea, is predominantly a black shale with minor amounts oflimestone. The shale is gray to black, hard, platy, pyritic,organic and very siliceous. The limestone is brown to darkbrown, microcrystalline to very finely crystalline, generallyclean, commonly sandy, and dolomitic (Bloom, 1988). InDodson Texas American Syndicate no. 1, the unit is 52 mthick (Stevens, 1957).The Mississippian limestone in Gaines and AndresCounties reaches 335 m thick. In Humble's Antelope Drawno. 1 it consists of over 76 m of crinoidal limestone, but inKalinga Corporation's Margaret no. 1, 9.6 km southeast ofthe Antelope Draw well, it is only 15 m thick. Thus, overall,Mississippian limestone thins southward over the study area.the end of the Silurian, Early Devonian, and Mississippianperiods.PennsylvanianThe history of the Delaware Basin as a separate eititybegan with the rise of an Early Pennsylvanian medial ridgein the Tobosa Basin, brought about by the Marat tionOuachita orogeny which was the dominant structural eventthat determined the nature of Pennsylvanian sediment ition(Galley, 1984). Movement along zones of weakness inheritedfrom Precambrian faulting (Fig. 9) induced the emergent ofa complex series of fault blocks near the center of the Tc bosaBasin (collectively known as the Central Basin Platform),which divided the Tobosa Basin into the Midland Basin onthe east and the Delaware Basin on the west (Figs. 10E and11C). In the southwestern part of the Central Basin Platform(near Fort Stockton, Texas), coarse, arkosic sandstone, varicolored shale, and subordinate carbonates of Pennsylvj nianBarnett Shaleage cover a large area of the shelf and directly overli; theOverlying the Mississippian limestone is an upper unit, Precambrian basement, the intervening Paleozoic ockscalled the Barnett Shale, consisting of partly silty, brown having been eroded away from the uplifted fault blockshale and containing very fine-grained sandstone and (Mazzullo, 1986). Also during this time tectonic activitysiltstone (Hayes, 1964; Wilshire et al., 1972; Grant and increased on the Diablo Platform to the west of the Dels wareFoster, 1989). The Barnett Shale is on the order of 60-140 m, Basin (Fig. 13), and these rocks were folded, faulted, andthickening somewhat to the west of the Pecos River. In some deeply eroded prior to the transgression of the Permiai sea.places the total thickness of Mississippian strata can reach The highlands which occupied the Apache Mountain area250-335 m (Netherland et al., 1974). In Kalinga subsequent to uplift were stripped down to Ordovician rock,Corporation's Margaret no. 1 well, the Barnett consists of 60 as can be seen in Humble's Reynolds Cattle Company "E no.m of shale with minor units of sandstone and limestone. The 1 well in the Apache Mountains where Permian Wolfe ampshale is gray to dark-gray towards the top, becoming dark- ian beds rest unconformably on the Ordovician Bliss Sandgray or black with depth; the shale is hard, siliceous, stone. In the Sierra Diablo west of the Apache Mountains,subfissile, organic, and very pyritic (Bloom, 1988).Wolfcampian beds rest on Precambrian rocks.In the area of Carlsbad, the top of the Mississippian belowThe Delaware Basin subsided rapidly duringthe "last" or lowest Pennsylvanian sand is called the Chester Pennsylvanian time. Material was eroded from the Pedernalmember, an informal unit which is composed of fine-grained uplift ("Ancestral Rockies"), Diablo Platform, Marathonsandstone, siltstone, argillaceous limestone, and shale Ouchita fold belt (Fig. 13), and other highland areas, and(Andersen, 1976). The Chester interval is usually considered was deposited as thin sequences of sand and shale withto be of Mississippian age, but may actually be the lowest interbedded carbonates on and along the edges of the shelves.part of the Pennsylvanian sequence. The Chester represents A deep, starved, shale basin occupied the central andthe first progradation of clastics from the Pedernal landmass southern parts of the broad Delaware depression throughoutand entrance into pre-delta deposition, and it signals the the Pennsylvanian, and during this period of geologic timebeginning of Pennsylvanian sedimentation in the basin.approximately 600 m of sediment accumulated in the basin.The Barnett Shale was the result of tectonic movement at Within the area of the Northwest Shelf, intrashelf basinsthe onset of the Ouachita orogeny (Gardiner, 1990). Sedi- (such as the Tatum Basin; Fig. 18) formed in the Latementation continued unabated in the Tobosa Basin until the Pennsylvanian as a result of regional tectonism, and theseLate Mississippian, but beginning at that time there was a persisted until the earliest Permian when the basins becametectonic upheaval that would, in the Pennsylvanian, divide filled (Grover. 1993)the Tobosa Basin up into the Midland Basin, Central BasinProvincial scries names for Pennsylvanian rock in thePlatform, and Delaware Basin (Fig. 11C). This ended the study area arc: the Morrowan, Alokan. Des Moinesian,existence of the Tobasa Basin, but during the 200 my or so Missourian. and Virgilian Equivalent formations (groups)from the Ordovician to Late Mississippian, this ancient basin are the Morrow. Atoka. Strawn. Canyon, and Cisco,accumulated as much as 2000 m of dolomite, limestone, respectively From a petroleum standpoint these names areshale, and sandstone — a continuous sedimentary record somewhat arbitrary, being based on a number of log tops andinterrupted only by relatively brief periods of exposure at the facies changes which are difficult to correlate from well toend of Early, Middle, and Late Ordovician time, and again at well (Grant and Foster. 1989) Meyer (1966) indicated a

39limestones, sandstones, shales, and siltstones (James, 1985).Over much of the area it is a fine- to coarse-grained andconglomeratic sandstone and gray shale with someinterbedded limestone and, in the west, it is a red shale andlimestone pebble conglomerate. In the Delaware Basinreference well located in Eddy County, the bulk of thePennsylvanian has been assigned to the Morrow; here, theMorrow interval i

Aug 10, 2006 · the Cable Canyon Sandstone and into the Cutter Formation, Aleman Formation, and Upham Dolomite, although normally these names are not used in the subsurface (Grant and Foster, 1989). The Montoya is 105 m thick in the Texas Richards well (T20S R32E) and thins to 88 m in the Honolulu Mako well (THS R28E). Along the Hovey anticline, northwest of