Thomas D. Fouch

Depositional Setting and Diagenetic Evolution of Some Tertiary Unconventional Reservoir Rocks, Uinta Basin, Utah

The Douglas Creek Member of the Tertiary Green River Formation underlies much of the Uinta basin, Utah, and contains large volumes of oil and gas trapped in a complex of fractured low-permeability sandstone reservoirs. In the southeastern part of the basin at Pariette Bench, the Eocene Douglas Creek Member is a thick sequence of fine-grained alluvial sandstone complexly intercalated with lacustrine claystone and carbonate rock. Sediments were deposited in a subsiding intermontane basin along the shallow fluctuating margin of ancient Lake Uinta. Although the Uinta basin has undergone postdepositional uplift and erosion, the deepest cored rocks at Pariette Bench have never been buried more than 9,800 ft (3,000 m). The sandstones, dominantly lithic arkoses and feldspathic litharenites, were derived from source terranes south of the Uinta basin. Secondary silica and several generations of authigenic calcite [Ca1.8-1.9(Mg0.02-0.06Fe0.02-0.06)(CO3)2], dolomite [Ca1.3-1.4(Mg0.6-0.7Fe0.02-0.04)(CO3)2], and ankerite [Ca1.2-1.3(Mg0.2-0.3Fe0.4-0.6)(CO3)2] form a replacive cement in the sandstones. Commonly, syntaxial overgrowths of late iron-bearing carbonate occur on detrital grains and preexisting relicts of iron-free carbonate cement. In sandstone where carbonate has been partly dissolved, abundant authigenic illite, partly ordered mixed- ayer illite-smectite, and small amounts of chlorite partly to completely fill secondary pores. Isotopic composition of carbonate cement and grain-supported rock range from -0.39 to -6.18 ^pmil for ^dgr13C and -7.80 to -13.98 ^pmil for ^dgr18O, indicating that authigenic carbonate formed at low temperatures in the presence of meteoric waters by a process of solution-precipitation. Enrichment of carbon and oxygen in early diagenetic calcite and fossiliferous rock relative to late diagenetic ankerite indicates a trend toward lighter isotopic carbonate compositions with increasing diagenesis. Kerogenous rocks at Pariette Bench are thermochemically immature and therefore are not the source of oil produced in the field. Hydrocarbons are compositionally similar to some of the oils produced from the Green River Formation in the Bluebell-Altamont field and are interpreted to have migrated from mature Green River source rocks through a network of open fractures. The occurrence of small amounts of hydrocarbon in secondary pores indicates that its emplacement postdated carbonate dissolution.

40Ar39Ar age constraints on neogene sedimentary beds, Upper Ramparts, half-way Pillar and Canyon village sites, Porcupine river, east-central Alaska

Abstract 40 Ar 39 Ar ages of volcanic rocks are used to provide numerical constraints on the age of middle and upper Miocene sedimentary strata collected along the Porcupine River. Intercalated sedimentary rocks north of latitude 67°10′N in the Porcupine terrane of east-central Alaska contain a rich record of plant fossils. The fossils are valuable indicators of this interior regions paleoclimate during the time of their deposition. Integration of the 40 Ar 39 Ar results with paleomagnetic and sedimentological data allows for refinements in estimating the timing of deposition and duration of selected sedimentary intervals. 40 Ar 39 Ar plateau age spectra, from whole rock basalt samples, collected along the Upper Ramparts and near Half-way Pillar on the Porcupine River, range from 15.7 ± 0.1 Ma at site 90-6 to 14.4 ± 0.1 Ma at site 90-2. With exception of the youngest basalt flow at site 90-2, all of the samples are of reversed magnetic polarity, and all 40 Ar 39 Ar age spectrum results are consistent with the deposition of the entire stratigraphic section during a single interval of reversed magnetic polarity. The youngest flow at site 90-2 was emplaced during an interval of normal polarity. With age, paleomagnetic and sedimentological data, the ages of the Middle Miocene sedimentary rocks between the basalt flows at sites 90-1 and 90-2 can be assigned to an interval within the limits of analytical precision of 15.2 ± 0.1 Ma; thus, the sediments were deposited during the peak of the Middle Miocene thermal maximum. Sediments in the upper parts of sites 90-1 and 90-2 were probably deposited during cooling from the Middle Miocene thermal maximum. 40 Ar 39 Ar results of plagioclase and biotite from a single tephra, collected at sites 90-7 and 90-8 along the Canyon Village section of the Porcupine River, indicate an age of 6.57 ± 0.02 Ma for its time of eruption and deposition. These results, together with sedimentological and paleomagnetic data, suggest that all of the Upper Miocene lacustrine sedimentary rocks at these sites were deposited during a single interval of reversed magnetic polarity and may represent a duration of only about 40,000 years. The age of this tephra corresponds with a late late Miocene warm climatic interval. The results from the Upper Ramparts and Half-way Pillar sites are used to estimate a minimum interval of continental flood basalt activity of 1.1–1.5 million years, and to set limits for the timing and duration of Tertiary extensional tectonic activity in the Porcupine terrane. Our data indicate that the oroclinal flexure that formed before the deposition of the basalts at the eastern end of the Brooks Range was created prior to 15.7 ± 0.1 Ma.

Miocene and Pliocene lacustrine and fluvial sequences, Upper Ramparts and Canyon village, Porcupine river, east-central Alaska

Abstract Cenozoic strata exposed along the Porcupine River between the Upper Ramparts and Canyon Village, Alaska, can be divided into five unconformity-bounded units (sequences) which are: lower and middle Miocene unit A, the white sandy fluvial sequence with peat beds; middle Miocene unit B, the basalt sequence—part B1 is basalt, and part B2 is organic-rich sedimentary beds; upper Miocene unit C, mudrock-dominated lake sequence; late Miocene or Pliocene to Pleistocene unit D, terrace gravels, detrital organic matter and associated sediments, and Holocene unit E, mixed sand and gravel-rich sediment and other sedimentary material including peat and eolian silt. The sequence (unit A) of lower and middle Miocene fluvial deposits formed in streams and on flood plains, just before the inception of local volanism. Fossil pollen from unit A suggests conifer-dominated regional forests and cool temperate climates. Peat beds and lake deposits from unit B contain pollen that indicates a warmer temperate climate coinciding with the middle Miocene thermal maximum. The lake deposits (unit C) downstream from the basalts accumulated in a small basin which resulted from a hydrologic system that was dammed in the late Miocene but breached soon thereafter. The lower part of the terrace gravels (unit D) expresses breaching of the dammed hydrologic system (of unit C). The Porcupine River became a major tributary of the Yukon River in late Pleistocene time when Laurentide ice blocked drainage from the Yukon interior basins causing meltwater to spill over the low divide separating it from the Porcupine River drainage initiating erosion and capture of the Yukon interior basins.

Depositional Environments, Diagenesis, and Hydrocarbon Potential of Nonmarine Upper Cretaceous and Lower Tertiary rocks, Eastern Uinta Basin, Utah: ABSTRACT

Core studies of nonmarine rocks from the Natural Buttes field, Utah, indicate that depositional environment and diagenetic alteration control the geometry and quality of low-permeability gas reservoirs in the eastern part of the Uinta basin. The Tuscher Formation (Upper Cretaceous) is composed of fine to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale that formed on the lower part of an alluvial braidplain. The Wasatch Formation (Paleocene and Eocene) unconformably overlies Cretaceous rocks and consists of fine-grained lenticular cross-bedded sandstones, argillaceous siltstones, and variegated mudstones, which were deposited in lower delta-plain settings along the margin of Lake Uinta. Cretaceous and Tertiary sandstones ave been modified by minor quartz overgrowths, by the precipitation and subsequent dissolution of ferroan and nonferroan calcite, by poikilotopic anhydrite, and by the formation of authigenic illite, mixed-layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate and anhydrite formed during early burial, before significant compaction. During later stages of diagenesis, precipitation of authigenic clay in secondary pores created by carbonate dissolution reduced porosity and permeability. Large amounts of natural gas generated in situ are stratigraphically trapped in these lenticular, diagenetically modified sandstones. Source rocks in the Tuscher Formation have reached the advanced stages of thermogenic gas generation (0.7% Ro) but are only moderatel mature with respect to liquid hydrocarbon generation. Interbeds of lacustrine Green River shale are in the early stages of gas generation (0.5% Ro) and are source rocks for gas produced from the Wasatch Formation. End_of_Article - Last_Page 860------------

Lacustrine Siliciclastic Rocks and Hydrocarbons: ABSTRACT

Siliciclastic rocks formed from sediments deposited in lake basins are source and reservoir units for large accumulations of oil and gas in North and South America, Africa, and Asia. Lacustrine strata of China consist primarily of siliciclastic units; those of Brazil, Angola, and Cabinda are principally of siliciclastic rocks with abundant carbonate beds; those of the United States consist of at least 50% carbonate rock. The sediments were deposited in, or peripheral to, ancient stratified lakes of a variety of ages, which for millions of years maintained a size comparable to that of modern inland seas. In siliciclastic fine-grained beds formed at the depositional center of these large lakes, values of organic carbon commonly average 3% or less. Those sequences of carbonate strata developed in open-lacustrine depositional settings commonly contain greater than 25% organic carbon. However, porosity values are generally very low in reservoir beds associated with the carbonate source rocks (generally cemented with carbonate minerals) relative to those associated with claystone source rocks. The worlds largest lacustrine oil and gas fields are developed in depositional systems with very thick sequences of open-lacustrine clay mudstone that contain relatively low values (generally 3% or less) of organic carbon. In these fields, hydrocarbons migrated from a thick central core of claystone into peripheral and overlying lacustrine and alluvial sandstones at relatively shallow burial depths and prior to significant compaction and cementation of the beds. Siliciclastic rocks form the principal reservoir units in most of the worlds ancient petroliferous lacustrine depositional systems. Lacustrine turbidite, bar, and deltaic sandstones are important reservoir units in Brazil, Africa, China, and the United States. Although nonmarine reservoir rocks are commonly described as being of a lacustrine origin, many were formed from sediment deposited at the edge of the lake or in settings well removed from the lake. A principal reservoir facies in the Uinta basin, Utah, is composed of siliciclastic beds that developed as the basal parts of coalesced fluvial channels at the fluctuating margin of Paleocene and Eocene Lake Uinta. Oil-bearing strata in some Chinese basins are channel-fill sandstones formed from sediment deposited in moderately sinu us streams on an alluvial plain several kilometers from the lacustrine shoreline. Depositional models of siliciclastic lacustrine rocks constructed to aid in the exploration and exploitation of indigenous hydrocarbon accumulations should be sensitive to type, richness, and thermochemical maturation of organic matter developed and preserved in the lake. Because siliciclastic open-lacustrine beds in many of the large petroliferous lake basins commonly contain relatively small amount of organic matter, thick sequences of such strata may be required to generate large accumulations of oil. Known accumulations are largest where migration has been to porous and permeable beds at relatively shallow burial depths. End_of_Article - Last_Page 462------------

Character of Ancient Petroliferous Lake Basins of the World: ABSTRACT

The principal oil- and gas-bearing lacustrine rocks in the world were formed from sediments deposited in or peripheral to ancient stratified lakes, of a variety of ages, which for millions of years maintained a size comparable to that of modern inland seas. In these lake systems, both lipid and woody organic matter were developed and preserved in large quantities. The lacustrine rock system commonly constitutes a depositional complex that includes indigenous hydrocarbon source, reservoir, and trap units. Lacustrine strata of China consist primarily of siliciclastic rocks; those of Brazil, Angola, and Cabinda are principally of siliciclastic rocks with abundant carbonate units; those of the End_Page 1680------------------------------ United States consist of at least 50% carbonate rock. Hydrocarbon resources of lacustrine depositional systems are greatest in the Peoples Republic of China (several billion barrels of recoverable oil). Hydrocarbon resources are also significant in the ancient lake basins of Brazil, Angola, Cabinda, and United States. Currently developed Chinese oil fields in nonmarine rocks are primarily in structural traps, those of South America and Africa are in combination structural and stratigraphic traps, whereas those of the United States are principally in stratigraphic traps. Available data suggest that hydrocarbons in the more deeply buried strata are contained in secondary pores which received oil or gas subsequent to significant episodes of cementation and/or compaction, and dissolution of minerals. Reservoir rocks with abundant primary porosity are most commonly preserved at relatively shallow burial depths, and many are intercalated with immature source units. Primary pores contain hydrocarbons that have migrated to reservoirs from mature source rocks (more deeply buried?). In China, oil is recovered in great quantities from sandstones with abundant primary porosity, particularly in those basins with high geothermal gradients. The oil apparently migrated to the primary pores from nearby source beds which reached thermochemical maturation at relatively shallow depths of burial and before significant early cementation and compaction of the sandstone units. Matrix porosity and permeability in sandstone units are best developed and preserved in those rocks composed of chemically stable minerals and few labile grains. Fluid-pressure gradients may be abnormally high in those lacustrine systems that have reached the stage of thermochemical maturation. In these cases, oil and/or gas are generated and expelled in quantities great enough to locally increase fluid pressures faster than pressure is released to adjoining rocks. Abnormally high fluid-pressure gradients in lacustrine units also occur in those impermeable hydrocarbon-bearing strata that have apparently been elevated at a rapid rate. In such a system, equilibration between fluid pressures in beds of low matrix permeability in the deep subsurface and permeable beds near the surface is restricted. In both cases, fractured overpressured rocks of low matrix permeability may yield oil and/or gas from pools whose boundaries are not restricted to loc l structures. Rather, they are restricted by relatively permeable beds that have provided access to the surface for pressure and fluid release (and invasion of water). Local avenues of permeability in overpressured rocks are greatest along natural, open fractures. Although reservoir rocks for fields developed in lake basins are commonly described as being of a lacustrine origin, others were formed from sediment deposited at the edge of a lake or in settings well removed from a lake. Principal reservoir rocks in the Uinta basin, Utah, represent the basal parts of coalesced fluvial channels formed at the fluctuating margin of lake Uinta. Red-colored oil-bearing strata in some Chinese fields whose reservoir rocks are channel-fill sandstones formed from sediment deposited on an alluvial plain several kilometers from the lacustrine shoreline. Lacustrine turbidite, bar, and deltaic rocks are important reservoirs in Brazil, Africa, United States, and China. Petroliferous sedimentary rocks formed in lake basins are known over much of the world where th y contain many billion barrels of recoverable oil and offer the promise of more. End_of_Article - Last_Page 1681------------