About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract


Pub. Id: A081 (1987)

First Page: 549

Last Page: 554

Book Title: SG 25: Exploration for Heavy Crude Oil and Natural Bitumen

Article/Chapter: Bitumen Deposits of Northwest Alabama: Section V. Exploration Histories

Subject Group: Oil--Methodology and Concepts

Spec. Pub. Type: Studies in Geology

Pub. Year: 1987

Author(s): G. V. Wilson

Abstract:

Bituminous rocks of Mississippian age crop out in northwest Alabama within a 113-km (70-mi) long belt. Bitumen deposits have been confirmed by coreholes to be present in the subsurface for a distance of at least 16-24 km (10-15 mi) south of the outcrop. Geochemical analyses indicate the bitumen to be an immature oil with an average sulfur content of less than 2%.

The Hartselle Sandstone is considered to have the best potential for future oil extraction operations. Saturations and thicknesses of the bitumen deposits in this formation vary widely with the "richest" deposits being located in the vicinity of west-central Lawrence County and southeast Colbert County. The highest measured saturation approaches 10% by weight and almost 70% of the pore space. Individual deposits up to 9 m (30 ft) in thickness have been encountered by coreholes, and multiple-impregnated zones are common throughout the Hartselle total thickness, which locally exceeds 46 m (150 ft). Estimates of the total reserves for the Hartselle are as large as 4.2 billion barrels (6.68 ^times 108 m3).

Text:

INTRODUCTION

Tar-sand deposits represent a potentially significant source of domestic oil to supplement the reserves of our nation. These deposits are primarily concentrated in nine states: Alabama, California, Kansas, Kentucky, Oklahoma, Missouri, New Mexico, Texas, and Utah.

The bitumen deposits of northwest Alabama have undergone commercial development only in a relatively small area of west-central Colbert County, where a bituminous sandstone and limestone have been surface mined for use in road construction. To date, there have been no attempts to establish commercial-scale, oil-extraction operations. The estimated reserves for the Hartselle Sandstone, however, and the characteristics and quality of the reservoir rocks in some areas, certainly indicate that serious consideration should be given to additional test-drilling and perhaps in situ pilot recovery projects.

GEOLOGIC SETTING

Rocks of Mississippian age crop out in an area that trends generally east-west through the Tennessee Valley region of north Alabama. These outcrop areas form the northern margin of the Warrior basin, which is bounded on the north by the Nashville Dome and on the east and southeast by the Appalachian fold and thrust belt. Dips are to the south and southwest at low angles, generally between 5.7 and 9.5 m/km (30 and 50 ft/mi). On the west and southwest, the rocks plunge beneath the cover of unconsolidated Mesozoic deposits of the Gulf Coastal Plain. The gentle southerly dips are unbroken by major faults and folds in or near the outcrop region, and only minor folds and faults are reported to be present (Kidd, 1980).

Throughout a period of more than 100 years, a number of authors have applied a variety of stratigraphic names to the outcropping Mississippian rock units. In the twentieth century, these authors have included Butts (1926); Welch (1958, 1978); and Thomas (1972; Fig. 1). Thomas (1972) divides the Mississippian strata of northwest Alabama into a lower unit consisting of extensive chert and cherty limestones (Fort Payne Chert and Tuscumbia Limestone), and an upper unit consisting of both carbonate and clastic sequences. Comprising the upper unit, in ascending order, are the Pride Mountain Formation, Hartselle Sandstone and Floyd Shale, Bangor Limestone, and the Parkwood Formation. The most widespread and significant bitumen deposits are found in the Pride Mountain Formation and Hartselle andstone.

The Pride Mountain Formation consists of a series of relatively thick units of shale with thin beds of limestone, sandstone, and siltstone. The section is dominated by medium- to dark-gray, fissile clay-shale that commonly contains abundant siderite nodules. The formation also includes calcareous shale, argillaceous limestone, and oolitic limestone beds that are often highly fossiliferous. Sandstone beds occur in the lower, middle, and upper parts of the formation, and these are generally lenticular in shape and often of limited areal extent. Bitumen deposits occur in the lower part of the Pride Mountain Formation and in both sandstone and limestone (Clark, 1925).

The Hartselle Sandstone is a fine-grained, quartzose sandstone that is cross laminated and mostly medium

End_Page 549------------------------

to thick bedded. It is the thickest and most widespread of all the sandstones of the Mississippian sequence in Alabama. The depositional environment of the Hartselle is believed to be that of a barrier island complex (Thomas, 1972). Sand deposits that locally exceed 46 m (150 ft) in thickness appear to trend in a northwest-southeast direction, and these linear features are separated by broad areas of thinner sand deposits (Chaffin et al., 1975).

OCCURRENCES AND PHYSICAL CHARACTERISTICS

Bituminous rocks are present in northwest Alabama within a 113-km (70-mi) long outcrop belt that extends from near the city of Hartselle in Morgan County westward through Lawrence County and into the western part of Colbert County near the Alabama-Mississippi boundary (Fig. 2). These bitumen deposits are found in both sandstones and limestones of Mississippian age, and they occur locally in the Pride Mountain Formation, Hartselle Sandstone, and Bangor Limestone. The oil saturations, thicknesses, and areal extent of these deposits vary widely, as do the porosities and permeabilities of the rocks in which they are found. The deposits are present under a variety of geologic and topographic conditions and beneath highly variable thicknesses and character of overburden. Detailed descriptio s of their locations and characteristics have been given in earlier reports (Clark, 1925; Jones, 1928; Wilson, 1983).

Bitumen deposits are more numerous and widespread within the Hartselle Sandstone, which commonly consists of tightly cemented, variably bedded, fine-grained, quartzose sandstones interbedded with varying thicknesses of shale. The sandstones are most often barren of hydrocarbons; however, a number of bitumen-impregnated zones are found within the outcrop area. Also, coreholes located up to 24 km (15 mi) south of the outcrop have encountered bituminous sandstone, thus indicating an area somewhat greater than 2590 km2 (1000 mi2) under which bitumen deposits are either known or likely to be present in varying degrees of thickness and quality. The results of coredrilling also indicate that oil-impregnated intervals of the Hartselle are more numerous than would be surm sed from the reported scattered occurrences in the outcrop area, wherein bedrock exposures are often quite limited in number, thickness, and areal extent.

Bitumen deposit thicknesses up to 5.5 m (18 ft) have been measured in the outcrop of the Hartselle Sandstone. Oil-impregnated zones up to 9 m (30 ft) thick have been encountered in coreholes. Multiple impregnated intervals have often been encountered in the testholes, and these are usually separated by sandstones barren of hydrocarbons. Bitumen saturations range widely from only a trace up to approximately 50 liters per metric ton (12 gallons per ton) and almost 70% of the pore volume. Porosities generally are 9-17%, but may be as much as 24% in thin zones. The permeabilities of these impregnated rocks vary widely, from less than one to several hundred millidarcies (Chaffin et al., 1975).

The lithofacies of the Hartselle Sandstone apparently play important roles in the accumulation and distribution of bitumen deposits. Higher oil saturations are generally found in the more porous and permeable "clean" sand facies of what has been interpreted to be northwest-southeast-trending bars or barrier island deposits (Thomas and Mack, 1982). Available core data south of the outcrop indicate that oil saturations increase from east to west (Chaffin et al., 1975). Analyses of surface samples show that the "richer" deposits are located in the area of west-central Lawrence County and southeast Colbert County (Wilson, 1983).

Bitumen deposits occur in both a sandstone and a limestone of the lower Pride Mountain Formation of west-central Colbert County. The sandstone has been referred to as the Bethel Sandstone (Butts, 1926), the Tanyard Branch Member (Welch, 1958, 1978), and simply the "lower sandstone member" (Thomas, 1972). It is a very fine-grained, slightly calcareous, thin- to medium-bedded, lenticular sand, which is generally less than 3.6 m (12 ft) thick and often displays abrupt variations in both thickness and lithology. The limestone, which lies stratigraphically about 7.6 m (25 ft) above the sandstone, is oolitic and fossiliferous, and the oil-impregnated interval generally ranges from 3.0 to 4.3 m (10-14 ft) in thickness. The average oil saturation for both the sandstone and limestone is probab y about 6 wt %. The areal extent of these deposits is unknown, but the sandstone is more restricted than the limestone deposits, which may underlie a 64.7-km2 (25-mi2) area and contain up to 120 million barrels of oil (Lewin and Associates, 1982). Both rock units have in the past been surface mined for use in road construction (Clark, 1925); however, no operations of this

Fig. 1. Stratigraphic names used for strata of Mississippian age in northwest Alabama.

End_Page 550------------------------

kind have been conducted since the late 1930s in the sandstone and since the late 1970s in the limestone.

The Bangor Limestone has been found to be bituminous in several areas along its outcrop and particularly in south-central and southwest Morgan County. Bangor porosities and oil saturations, however, are generally much lower than those measured for the oil-impregnated intervals of the Hartselle Sandstone and Pride Mountain Formation.

Oil saturation analyses have been conducted on outcrop samples of bituminous rocks from both the Hartselle Sandstone and Pride Mountain Formation (Table 1). Analyses of samples collected from the same outcrops have been conducted by various methods, and these show similarities in the relative grades of the deposits. Hartselle Sandstone oil saturations (samples 1-10), determined by Soxhlet extraction methods, range from 2.49 to 9.66% by weight and averaged 6.15%. The highest saturations are found in samples collected from outcrops located in northwest Lawrence County (samples 5 and 6) and southeast Colbert County (samples 7 and 8).

The bitumen displays a highly variable degree of mobility, which is primarily dependent upon the degree of saturation and the lighter hydrocarbon content. Results of Fischer Assay analyses (Table 1) show a significantly higher percent of gas loss for samples 5 and 7, thus indicating a greater abundance of lighter hydrocarbon fractions at these localities. Other indications of higher oil saturations and the presence of lighter hydrocarbon fractions in some local areas include oil seepages in outcrops along fractures and bedding planes and some reported occurrences of the forced abandonment of water wells as the result of contamination by oil seepage.

CHEMICAL CHARACTERISTICS

Geochemical analyses of bitumen samples extracted from the Hartselle Sandstone and Pride Mountain Formation indicate that these are immature rather than mature oils (Table 2). The paraffin-naphthene/ aromatic hydrocarbon ratios (P-N/AROM) for four samples ranged from 0.53 to 0.87. These values are significantly lower than the 1.2 or greater value that would be representative of a mature crude oil. Also,

Fig. 2. Hartselle Sandstone outcrop and tar sand localities in northwest Alabama.

End_Page 551------------------------

the average hydrocarbon percentage for the four samples was approximately 50%, and the dominant saturated hydrocarbon was found to belong to the naphthene series, which ranged from 90.9 to 98.7% of the samples. Available gas chromatograms (Chaffin et al., 1975) show a well-developed, bimodal naphthene peak distribution with strong preference toward the 25, 26, and 27 peaks that represent tetracyclic and pentacyclic naphthenes. Terpanes and steranes are indicated to be present by the characteristics of the naphthene envelopes in the vicinity of these peaks, and this is a further indication of immaturity.

Ultimate analyses (Table 3) have been performed on the bitumen extracted from rock samples collected at 12 outcrop localities (Wilson, 1983). Ten of these samples were from the Hartselle Sandstone (samples 1-10), and one each from the sandstone and limestone of the Pride Mountain Formation (samples 11 and 12, respectively). The carbon content of the ten bitumen samples extracted from the Hartselle ranged from 78.58 to 83.27% and averaged 81.35%, and the hydrogen ranged from 9.69 to 10.60% and averaged 10.16%. The nitrogen content of these same ten samples ranged from 0.55 to 0.90% (average of 0.78), and the sulfur content ranged from 1.18 to 2.02% (average of 1.71).

RESERVE ESTIMATES AND RECOVERY POTENTIAL

The Hartselle Sandstone has been estimated to contain 0.7 billion barrels of bitumen within the outcrop area and 2.2-3.5 billion barrels in the subsurface (Lewin and Associates, 1982). This estimate for the outcrop is based on an assumed area of 906 km2 (350 mi2) and an average thickness of 2.4-3.0 m (8-10 ft). The subsurface area was assumed to be 2590 km2 (1000 mi2) with an average net pay thickness of 5.5 m (18 ft). The in-place oil resources for the Pride Mountain Formation in outcrop have been estimated to be 0.12 billion barrels, assuming an areal extent of 65 km2 (25 mi2), an average net pay thickness of 3.7 m (12 ft), and an average oil content of 6%.

Reserve estimates for the Hartselle Sandstone have been made only for an area that includes parts of Colbert, Franklin, Lawrence, and Morgan counties, for it is only in these areas that at least some corehole data are available to perform reserve calculations. The southern or downdip limits of the bitumen deposits have not been defined by test drilling, and large areas remain to be explored in these counties and also in Marion, Winston, and Cullman counties to the south. It is possible that future test drilling may raise the reserve estimates to more than 5 billion barrels.

Surface mining of the bitumen deposits of northwest Alabama may be conducted on a relatively small or moderate scale in the future. The mostly lean, relatively thin, and somewhat discontinuous nature of these deposits, however, will likely prohibit any large-scale surface mining of the rocks for oil extraction purposes. The deposits have been classified as mineable, by both surface and underground techniques; however, this resource is presently considered to be marginal in the outcrop area because of its average low grade and lateral discontinuities (Resnick et al., 1981).

Available data indicate that any future large-scale commercial extraction operations will likely be by in

Table 1. Bitumen saturations of outcrop samples by various methods.

End_Page 552------------------------

Table 2. Summary of C15+ Soxhlet extraction, deasphaltening, and liquid chromatography of bituminous rock samples from northwest Alabama.

Table 3. Ultimate analyses of bitumen extracted from outcrop samples collected in northwest Alabama.

End_Page 553------------------------

situ methods. Laboratory tests have demonstrated that thermal recovery techniques could be utilized (Moftah, 1973). The prospects for the eventual economic recovery of the bitumen by in situ methods are made brighter by the apparent increase in the average thickness of oil-impregnated intervals in a downdip direction (a condition not well understood); the existence of multiple-impregnated zones downdip; and a possible downdip increase in oil mobility.

A thorough evaluation of the economic potential of the bitumen deposits of northwest Alabama must await the gathering of additional data, primarily from core-drilling projects designed to determine the regional downdip limits of the deposits and, in selected areas, to determine the average saturation, thickness, continuity, and extent of individual deposits. Finally, in these selected areas, pilot recovery projects utilizing the most advanced techniques for in situ recovery must be conducted in order to evaluate fully the importance and economic potential of these deposits.

References:

Butts, C., 1926, Paleozoic rocks, in G. I. Adams, et al., Geology of Alabama: Alabama Geological Survey Special Report 14, p. 162-206.

Chaffin, H. S., Jr., W. M. Beavers, and P. A. Boone, 1975, Petroliferous rocks (Mississippian age) of north Alabama: U.S. Bureau of Mines Open-File Report 21-76, 191 p.

Clark, G. H., 1925, Rock asphalts of Alabama and their use in paving: Alabama Geological Survey Special Report 13, 96 p.

Jones, W. B., 1928, Summary of the distribution and occurrence of the rock-asphalt deposits of Alabama: Alabama Geological Survey Circular 5, 19 p.

Kidd, J. T., 1980, General geology, geophysics, and seismicity of northwest Alabama: Prepared for the Division of Reactor Safety Research, Office of Nuclear Regulatory Commission by the Alabama Geological Survey: NUREG/CR-1519, 79 p.

Lewin and Associates, 1982, Tar sand and shallow oil fields of Alabama: Prepared for the Interstate Oil Compact Commission (IOCC), 25 p.

Moftah, I., 1973, Engineering evaluation of Alabama tar sands: Alabama Geological Survey Circular 89, 58 p.

Resnick, B. S., D. H. Dike, L. M. English, III, and A. G. Lewis, 1981, Evaluation of tar sand mining. Volume 1. An assessment of resources amenable to mine production: Prepared for U.S. Department of Energy by Ketron, Inc.: DOE/ET/30201-1 (DE 82010249), p. 49-53 and p. 141-142.

Thomas, W. A., 1972, Mississippian stratigraphy of Alabama: Alabama Geological Survey Monograph 12, 121 p.

Thomas, W. A., and G. H. Mack, 1982, Paleogeographic relationship of a Mississippian barrier-island and shelf-bar system (Hartselle Sandstone) in Alabama to the Appalachian-Ouachita orogenic belt: Geological Society of America Bulletin, v. 93, n. 1, p. 6-19.

Welch, S. W., 1958, Stratigraphy of the Upper Mississippian rocks above the Tuscumbia Limestone in northern Alabama and northeastern Mississippi: U.S. Geological Survey Oil and Gas Inventory Chart OC-58.

Welch, S. W., ed., 1978, Mississippian rocks of the Black Warrior basin: Guidebook for the seventeenth field trip of the Mississippi Geological Society, April 20-21, 1978, 62 p.

Wilson, G. V., 1983, Physical and chemical characteristics of Alabama tar sands: Prepared for the Tennessee Valley Authority by the Geological Survey of Alabama: TVA/OP/EDT-83/11, 93 p.

End_of_Record - Last_Page 554-------

Copyright 1997 American Association of Petroleum Geologists

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24