James A. Peterson

Pennsylvanian Evaporite-Carbonate Cycles and Their Relation to Petroleum Occurrence, Southern Rocky Mountains

The Paradox and Eagle basins of southeastern Utah and western Colorado contain a thick series of cyclic evaporite deposits of Pennsylvanian (Desmoinesian) age. In response to marked salinity gradients, halite and potash were deposited in the deeper parts of these basins, whereas on the basin margins, or shelves, carbonates were the predominant facies. Thus each evaporite cycle of the inner basin has a shelf counterpart in the carbonate facies. Each cycle has its own particular facies pattern, which is a reflection of biologic and chemical response to changes in sea level. Within the shelf carbonate facies some cycles contain lens-shaped buildups of biogenic carbonates. These carbonate mounds form the productive reservoir rocks on the southwest shelf of the Paradox basin. imilar, but as yet unproductive and relatively untested, carbonate mounds are present in the Eagle basin. Most of the petroleum production in the Paradox basin is from porosity development associated with moundlike buildups of algal and leached oolite limestones. Aneth, the major field on the southwest shelf of the basin, contains an estimated 300 million bbl of oil. Some production comes from the inner-basin evaporite facies from thin beds of intensely fractured black shale and dolomite. The Paradox basin is only one of the many areas of the world where petroleum resources are associated with evaporites. A close appraisal suggests that the evaporite environment plays a direct role in generation and accumulation of petroleum. Barred basins, which probably have contributed the greatest volume of marine evaporites, not only concentrate oceanic salts but also form an efficient trap for organic matter. In addition, much of this organic matter is in solution, which may facilitate its conversion to petroleum. Evaporation losses in a large basin create a strong basinward flow of nutrient-rich water. If this water moves across a shallow shelf, it may stimulate the activities of the shelfs biologic community and accelerate production of organic matter. Anoxic conditions within the evaporite basin, caused by low solubility of oxygen in high-salinity brines, combined with high levels of H2S produced by biogenic sulfate reduction, retard the decay process and allow preservation of most of the organic matter swept in from the shelf.

Paleotectonics and sedimentation : in the Rocky Mountain Region, United States

The main basins and uplifts of the Rocky Mountain region are outlined by a generalized thickness map of the total Phanerozoic sedimentary cover. Twelve maps showing thickness, generalized sedimentary facies, and probable emergent terrigenous clastic source areas are presented, covering the late Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, and Cenozoic.

Stratigraphic vs. Structural Controls on Carbonate-Mound Hydrocarbon Accumulation, Aneth Area, Paradox Basin

Pennsylvanian oil and gas accumulations in the southern Paradox basin are in carbonate mounds of Desmoinesian age. Major oil production at Aneth, Ismay, Tohonadla, Gothic Mesa, Anido Creek, and other fields is from algal mounds elongate in a general northwest-southeast direction along the basin shelf. Carbonate reservoirs are associated closely with sapropelic black shale and evaporite, which occur in cyclic repetition in the shelf area and grade basinward to a predominantly salt section. All reservoirs appear to be isolated bodies of porous carbonate, mostly limestone. About 30 oil and gas fields productive from the Pennsylvanian have been found in the Four Corners area; approximately half are classified as stratigraphic and the other half as either structural or structural-stratigraphic. In almost all fields, it can be demonstrated that the accumulation would have formed even if no structural closure were present, although in many fields the oil is localized by subsequent structural growth. The Aneth, Ismay, and Cache fields are primary examples of Paradox basin fields showing major stratigraphic and only minor structural influence on accumulation.

Marine Jurassic of Northern Rocky Mountains and Williston Basin

Regional studies indicate that the pattern of sedimentation within the Jurassic seas of the northern Rocky Mountains was influenced by several associated paleotectonic elements. The major negative-trending units were the Alberta and Twin Creek troughs and the Williston basin. The major positive-trending elements were the Belt island and the Sheridan arch. Regionally the marine Jurassic can be separated into lithogenetic units whose characteristics appear to be directly related to the presence of sedimentary environments, the nature of which was governed by the position and relative activity of the various paleotectonic elements. The Piper and Rierdon formations and their equivalents are each separated into three regional units of member rank. The lithologic nature of these units is believed to be related in a large way to the position of Belt island and its effect upon the salinities, temperature, and current distribution in the waters of the Twin Creek trough and the Williston basin and the intervening Montana-Wyoming shelf area. Much of the misunderstanding concerning the Jurassic stratigraphy of the Williston basin is believed to be caused by an inadequate knowledge of the regional nature of the Jurassic units.

Sedimentary History and Economic Geology of San Juan Basin

The San Juan basin contains up to 15,000 feet of sedimentary rocks ranging in age from Cambrian to Recent. Beginning with the Cambrian transgression, at least 10 major events in the sedimentary history of the basin area can be recognized. The earliest development of the area as a sedimentary basin or trough apparently took place in Pennsylvanian time and the basin was maintained, with changing rates of subsidence and filling, through the remainder of geologic time. During the early Paleozoic, sedimentation was dominated by marine transgressions across the northwestern flank of the regional Transcontinental arch. The late Paleozoic history was strongly influenced by tectonism related to development of the Ancestral Rocky Mountains uplifts and associated downwarping. Domina tly cyclic marine carbonate deposition during the early phases of this event preceded the infilling of the trough with coarse clastics. The early Mesozoic is characterized by fluvial and eolian environments, interrupted periodically by thin marine transgressive deposits of nearshore redbeds. The final Mesozoic event was the widespread Upper Cretaceous marine transgression which deposited a thick cyclic sequence of marine gray shale and sandstone, with interbedded coal beds. Final withdrawal of marine waters in Late Cretaceous time was associated with the rise of major uplifts on the northern and eastern sides of the basin and infilling of the basin area with stream, lacustrine, and paludal deposits during early Tertiary time. Late Teritary regional uplift and resulting volcanism were acc mpanied by a regional dissection of the area by stream systems that evolved into the present-day drainage pattern of superposed streams. The sedimentary history is directly related to the occurrence of economic deposits of the basin. Major reserves of petroleum are in Cretaceous and Pennsylvanian sections, coal in Cretaceous, and uranium in Jurassic and Cretaceous.

Zuni Sequence in Williston Basin--Evidence for Mesozoic Paleotectonism

The Zuni sequence in the Williston basin is a large-scale lithogenetic package bounded by interregional unconformities. Within the sequence, three major subdivisions are separated by unconformities or marker beds and correspond with chronostratigraphic units: (1) Middle and Upper Jurassic, (2) Lower Cretaceous, and (3) Upper Cretaceous and Paleocene. The basin has clear expression in the Jurassic subdivision, poor expression in the Lower Cretaceous, and good expression in the Upper Cretaceous. Jurassic units are thick in the basin center, thin in central Montana and northeastern Wyoming, and thick in southern Montana and western South Dakota. Jurassic marine carbonates are found along the western basin margin and marine sandstones mark the southern and eastern margins. Lower Cretaceous rocks display a regional thinning from west to east with little expression of the basin center and margins. Lower Cretaceous marine and nonmarine sandstones form blanket deposits. Upper Cretaceous units preserved below the Paleocene show a clearly defined depocenter at basin center. Upper Cretaceous shales characterize the depocenter; facies patterns of marine sandstones on the west and south and carbonates on the east correspond with paleotectonic elements distributed around the basin margin . A series of seven marginal paleotectonic elements surround the basin center on the west, south, and east in the United States. Five more marginal elements have been described in Canada. Occurrences of oil in the Jurassic and Lower Cretaceous and of natural gas in the Upper Cretaceous are broadly related to the pattern of marginal paleotectonic elements.

Petroleum Geology and Resources of Southeastern Mexico, Northern Guatemala, and Belize: ABSTRACT

Petroleum deposits in southeastern Mexico and Guatemala occur in two main basinal provinces: the Gulf Coast Tertiary basin area, which includes the Reforma and offshore Campeche Mesozoic fields, and the Peten basin of eastern Chiapas State (Mexico) and Guatemala. Major oil production, in order of importance, is from Cretaceous, Paleocene, and Jurassic carbonate reservoirs in the reforma and offshore Campeche area. Several small oil fields have been discovered in Cretaceous carbonate reservoirs in west-central Guatemala. Almost all important production is in salt structure traps or on domes and anticlines that may be related to deep-seated salt movement. Some minor oil production has occurred in Cretaceous carbonate reservoirs in a buried overthrust belt along the west flank of the Veracruz basin. Gas production is mainly from Tertiary sandstone reservoirs.

Subsurface Stratigraphy and Depositional History of Madison Limestone (Mississippian), Williston Basin: ABSTRACT

Cyclic carbonate-evaporite deposits of the Madison Limestone (Mississippian) in the Williston basin are made up of four main facies. From basin to shelf, the normal facies transition is from offshore deeper water (Lodgepole) facies to crinoidal-bioclastic banks at the basin to shelf transition, to oolite-algal banks and back-bank fine carbonate, evaporite, and minor terrigenous clastic beds on the shallow shelf. Five major depositional cycles are correlated and mapped on the basis of shaly marker beds identified on gamma-ray-neutron or gamma-ray-sonic logs. The marker beds are interpreted as reworked and redistributed silt and clay-size sediments originally deposited, possibly by eolian processes, on the emergent shelf during low sea level phases of cycle development. Fro oldest to youngest, the first two cycles are characterized by increasing amounts of crinoidal-bioclastic and oolite-algal carbonates, culminating in the Mission Canyon facies of the middle cycle. The upper two cycles are characterized by increasing amounts of evaporite deposits, culminating in the Charles salt facies of the youngest cycle. Much of the Madison section on the south and east flanks of the basin consists of dolomite. Dolomite content decreased toward the basin center, where a major share of Madison petroleum production is located. Reservoir beds in the oil fields are primarily partially dolomitized oolite-algal or crinoidal-bioclastic bank carbonates. Most of the productive petroleum reservoirs are located in the middle cycles of the Madison. End_of_Article - Last_Page 859------------

West Siberian Basin: ABSTRACT

The West Siberian basin (~3.4 × 106 sq km) is one of the largest structural-sedimentary basins of the world. The basin was relatively undisturbed by post-Triassic tectonism and erosion and is little changed from its original form when Early Jurassic deposition began about 180 m.y. ago. The large Khanty structural high in the central part of the West Siberian basin is nearly 1,000 km long and 400 km wide. The axis bears five large domes, separated by depressions. The Khanty arch was a structural entity throughout the Mesozoic and is quite clearly the locus of greatest oil occurrence. Although the depositional history of the basin was one of continual incursion and retreat of the sea, three megarhythms are recognized in the sedimentary fill: Triassic-Aptian, Aptian-Oligocene, and Oligocene-Quaternary. Continental sediments predominate at the base of each megarhythm, and largely marine and nearshore sediments are present at the top. Megarhythms are made up of macro-, meso-, and microrhythms, each of which has its transgressive and regressive phases. The search for paleoshorelines and related stratigraphic traps seems to be yet in an early stage. Three major productive areas are recognized in the basin. In the west, near the Ural Mountains, oil and gas are produced from Upper Jurassic sandstones that pinch out against basement blocks. Along the middle End_Page 432------------------------------ course of the Ob River in the central part of the basin, production is largely oil from Lower Cretaceous arkosic sandstones on anticlines. Samotlor, the largest oil field in the USSR, is in this area. In the northern part of the basin, mainly gas is produced from Upper Cretaceous clastic rocks on anticlinal traps. Urengoy, the worlds largest gas field, is located in this area. End_of_Article - Last_Page 433------------

Pennsylvanian Evaporite Cycles And Petroleum Production, Southern Rocky Mountains: ABSTRACT

Cyclic evaporite deposits of Pennsylvanian age are associated with carbonate-mound petroleum reservoirs in the Paradox and Eagle basins of southeastern Utah and western Colorado. In both basins there are basin-type evaporites associated with starved-basin trough sediments that cover a complete range of evaporitic deposits. A broad facies belt of petroleum-bearing biogenic carbonates and fine clastics occurs in cyclic association on the mildly tectonic basin shelves. Five major juxtaposed evaporite-carbonate mound cycles are mappable in the Paradox basin. The evaporite environment is believed to be directly responsible for the presence of petroleum deposits in the Paradox basin. Most of the petroleum deposits occur in shelf carbonate reservoirs along the SW. flank of the basin. (Longer abstract available)