Robert R. Berg

Capillary Pressures in Stratigraphic Traps

Capillary pressures between oil and water in rock pores are responsible for trapping oil, and the height of oil column, zo, in a reservoir may be calculated from the Hobson equation modified as follows: [EQUATION] where ^ggr is the interfacial tension between oil and water, rt is the radius of pore throats in the barrier rock, rp is the radius of pores in the reservoir rock, g is acceleration of gravity, and ^rgrw and ^rgro are the densities of water and oil, respectively, under subsurface conditions. To apply the equation, pore sizes must be estimated from mean effective grain sizes of the reservoir and barrier rocks. Effective grain size, De, in centimeters can be approximated from core analysis data by means of an empirical permeability equation from which [EQUATION] where n is porosity in percent and k is permeability in milli-darcys. Then pore and throat sizes may be estimated as functions of mean effective grain size as based on theoretical packings of grains. The oil-column equation assumes hydrostatic conditions, and additional column, ^Dgrzo, may be trapped if hydrodynamic flow occurs down the dip from barrier to reservoir facies, or [EQUATION] where dh/dx is the potentiometric gradient and xo is the width of the oil accumulation. Calculations of oil columns in stratigraphic fields show that the equations give values which are in fair to good agreement with observed oil columns; that porous and permeable, very fine-grained sandstones and siltstones are commonly effective barriers to oil migration; and that the recognition of such barriers can be important in exploration for stratigraphic traps.

Primary Migration by Oil-Generation Microfracturing in Low-Permeability Source Rocks: Application to the Austin Chalk, Texas

Fracturing of low-permeability source rocks is induced by pore-pressure changes caused by the conversion of organic matter to less dense fluids (oil and gas); these fractures increase the permeability and provide pathways for hydrocarbon migration. An equation for the pressure change is derived using four major assumptions. (1) The permeability of the source rock is negligibly small (0.01 µd; 10-20 m2) so that the pore-pressure buildup by the conversion is much faster than its dissipation by pore-fluid flow. (2) The stress state is isotropic so that horizontal and vertical stresses are equal. The source rock fails when the pore pressure equals the overburden pressure. (3) The properties of the rock, organic matter, and fluids remain constant during oil generation. This assumption is valid when the change in depth (i.e., pressure and temperature) is small. (4) Only two reaction rates are required for the conversions, a low-temperature reaction rate for the kerogen/oil conversion (E approx. = 24 kcal/mol, A approx. = 1014/m.y.) and a high-temperature reaction rate for oil/gas conversion (E approx. = 52 kcal/mol, A approx. = 5.5 ´ 1026/m.y.). The equations for generation rate and pressure change are applied to the Austin source rock by adjusting the several variables to fit geochemical data, core saturations, and observed levels of oil and gas production. This application demonstrates that the equations are easily applied in calculating depths of primary migration for low-permeability source rocks.

Mountain Flank Thrusting in Rocky Mountain Foreland, Wyoming and Colorado

The principal mountain ranges of the Wyoming and Colorado foreland were raised asymmetrically by Laramide uplift which began in latest Cretaceous time as dominantly vertical movement along arcuate trends. Uplift continued into the Paleocene, and the steeper flanks of some ranges developed into large overturned folds by local compressive forces marginal to the main uplifts. Along segments of maximum uplift, overturned folds were broken and thrust far over the basin synclines in latest Paleocene or earliest Eocene time. Throughout the long period of uplift the adjacent basins were downwarped continuously and received sediment from the rising mountains. The process of uplift by folding and thrusting better explains observed structure of mountain flanks than the older ideas of block uplift along high-angle faults or thrust uplift by regional compression. Fold-thrust structures are best known along the major Wyoming thrust zones bordering, on the south, the Wind River and Granite Mountains and the Washakie and Owl Creek Mountains. Other examples of faulted overturned folds occur in Colorado along the Golden thrust of the Front Range and Willow Creek thrust in western Colorado. In all these areas thrusts are well documented by subsurface control which includes both deep wells and seismic data.

Recognition of Barrier Environments

The vertical succession of sedimentary structures and textures at Galveston Barrier Island, Texas, is identical with vertical successions in two ancient barrier complexes, one in the Lower Cretaceous of Montana and the other in the Lower Jurassic of England. Within both Holocene and ancient examples, there is a gradation upward from (1) irregular interlaminations of siltstone and claystone at the base, through (2) burrowed and generally structureless sandstone, to (3) low-angle and microtrough cross-laminated sandstone, terminating in two of the examples in (4) structureless and rooted sandstone. This sequence represents deposition in (1) lower shoreface, (2) middle shoreface, (3) upper shoreface-beach, and (4) eolian environments, respectively. Analyses of quartz size and content of the Holocene and ancient barriers yield textural and compositional parameters that are environmentally sensitive. Plots of these parameters demonstrate that each of the environments may be distinguished on the basis of thin-section analyses. Consequently, full diameter cores, which show sedimentary structures, may not be necessary for precise environmental interpretation in the subsurface. Indeed, thin sections of sidewall cores may yield significant and reliable environmental interpretations in barrier sandstones. Textural and sedimentary structural similarities between Galveston Island and the ancient examples permit a general model of barrier sedimentation to be developed.

Sealing Properties of Tertiary Growth Faults, Texas Gulf Coast

Growth faults consist of nonsealing fault surfaces and sealing sheared zones that may occur on either the footwall or hanging wall. The properties of sheared zones are assumed to be identical to those of soft sediment that has undergone ductile deformation during mass movement. In cores, the sheared zones display fabrics similar to Riedel shears and are termed wispy, crenulate, conjugate, and meniscate, in order of increasing deformation. Permeabilities and porosities range from 0.1 md and 18% to less than 0.01 md and 8%. Based on limited measurements, initial mercury-injection capillary pressures range from 400 to 550 psia, sufficient to trap an average oil column of 98 m (320 ft). Sheared zones are effective seals because ductile deformation has homogenized the original sediments and resulted in a uniform distribution of small pores. In contrast, the fault surface is a region of extension that is presumed to result in higher permeabilities, low displacement pressures, and the ability to transmit migrating oil and gas from deep source beds to shallow traps. Thus, growth faults can seal in the sheared zone and leak along the fault surface. Sheared zones are distinctive on dip logs. Dips within sheared zones have variable magnitudes and directions, whereas dips adjacent to faults exhibit more uniform patterns resulting from normal drag.

FRANCONIA FORMATION OF MINNESOTA AND WISCONSIN

The Upper Cambrian Franconia formation in southeast Minnesota and west-central Wisconsin consists of glauconitic, quartzose sandstones that average 175 feet in thickness. Previous subdivision of the Franconia resulted in faunal zones to which geographic member names were given. These zones are not rock units and cannot properly be called members. In this paper, members are based on rock type. They are, in ascending order, the Woodhill member—medium- to coarse-grained sandstone; the Birkmose member—fine-grained, glauconitic sandstone; the Tomah member—sandstone and shale; and the Reno member—glauconitic sandstone. A fifth unit, the Mazomanie member—thin-bedded or cross-bedded sandstone, forms a nonglauconitic facies that interfingers with and replaces the Reno member. Faunal zones are independent of the lithic units. The Woodhill member was deposited in the transgressing Franconia sea. Birkmose greensands formed in shallow waters far from shore, while Tomah sand and shale and Mazomanie thin-bedded sand were deposited nearer shore. Later, Reno greensands formed offshore, Mazomanie thin-bedded sand was deposited shoreward, and cross-bedded Mazomanie sand accumulated nearest the strand line.

Hydrodynamic Effects on Mission Canyon (Mississippian) Oil Accumulations, Billings Nose Area, North Dakota

Mission Canyon oil production on the south flank of the Williston basin provides an example of an area in the mature stage of exploration that shows significant hydrodynamic effects on oil accumulations related to stratigraphic traps. The effects are illustrated by the Billings Nose fields and the Elkhorn Ranch field. The reservoirs have low hydraulic gradients of about 2 m/km (10 ft/mi), tilted oil-water contacts with gradients of 5 m/km (25 ft/mi), and variable formation-water salinities that range from brackish to highly saline. Oil accumulations in some zones are displaced off structure and downdip to the northeast, parallel to porosity pinch-outs. Other zones are pure hydrodynamic traps, lacking both structural and stratigraphic closure. Future success in exploration and development in the play will depend on recognizing the hydrodynamic effects and predicting oil displacement.

Depositional Environment of Upper Cretaceous Sussex Sandstone, House Creek Field, Wyoming

Sussex sandstone produces oil in a stratigraphic trap at House Creek field in the south-central Powder River basin. The total thickness of the Sussex is 40 ft (12 m) and reservoir sandstone is as much as 30 ft (9 m) thick, about 1 mi (1.6 km) wide, and 25 mi (40 km) long. The sandstone is probably of marine origin but has an unexpected bedding sequence: (1) an upper bioturbated mudstone that contains scattered granules of chert; (2) thin, coarse-grained, pebbly sandstone; (3) thin-bedded, cross-laminated sandstone with deformed shale clasts; (4) ripple-bedded sandstone with shale laminae; and (5) a basal shale that contains ripple lenses of sandstone and rare bioturbation. Mean size of quartz is 0.22 mm (fine grained), and grain size increases upward from 0.13 mm at the b se to 0.28 mm in the pebbly sandstone. The sandstone contains monocrystalline quartz, 48 percent; feldspar, 11; polycrystalline quartz, 10; chert, 13; and clay matrix, glauconite, and chlorite, 15 percent. The Sussex represents a prograding sequence of sand that was transported southeastward by low-flow-regime currents along a relatively shallow, marine shelf. Bioturbation, stratigraphic position, and similarity to other sandstones in the same sequence suggest a neritic environment of water depths in the range of 100 to 200 ft (31 to 62 m). The thin, pebbly sandstone is not fully explained because its coarse texture suggests stronger, perhaps channelized, currents. Nevertheless, the narrow, prograding sequence probably represents a major transport route for sand across the shelf toward a deeper basin beyond.

Petrography and Origin of Lower Tuscaloosa Sandstones, Mallalieu Field, Lincoln County, Mississippi: ABSTRACT

Upper Cretaceous sandstone of the lower Tuscaloosa Formation in southwestern Mississippi is part of a fluvial-deltaic depositional system. At the Mallalieu field, lower Tuscaloosa sandstone is of two types: (1) channel-fill sandstone--thin, lenticular bodies which have irregular distribution across the field; and (2) point-bar sandstone--thick, more continuous bodies which have a ridge-and-swale pattern of sandstone distribution and which laterally are terminated End_Page 1828------------------------------ abruptly by narrow, broadly arcuate, shale-filled channels. These two fluvial interpretations are supported by mineralogy, textural gradation, internal structures, and sand-body geometry. The average composition of lower Tuscaloosa sandstone is quartz, 60 percent; matrix, 32 percent; calcite cement, 4 percent; feldspar, 1 percent; muscovite, 1.5 percent; and other minerals, 1.5 percent. Average mean grain size of quartz is 0.24 mm (fine grained); mean grain size decreases upward within individual sandstone beds. Four distinct sandstone zones produce oil at Mallalieu. The lower two zones are characterized by more extensive, point-bar sandstone whereas the upper two zones are narrow, channel-fill sandstone. This vertical sequence suggests an upward gradation from fluvial meander-belt deposition, through deltaic distributary deposition, to inner neritic deposition of the overlying marine shale--an overall transgressive sequence. The change from meandering below to braided above probably resulted from a change in stream gradient by basin subsidence. End_of_Article - Last_Page 1829------------

Point-Bar Origin of Fall River Sandstone Reservoirs, Northeastern Wyoming

Proved oil reserves of more than 40 million bbl have been found in the Lower Cretaceous Fall River Sandstone on the northeast flank of the Powder River basin. Most of the oil is in stratigraphic traps on gentle regional dip of about 2° SW. Fall River reservoir sandstone formerly was believed to have been deposited as barrier bars with the updip permeability barriers provided by lagoonal shale. A new interpretation of West Moorcroft field (Mettler, 1966) suggested an origin as point-bar sand deposited in a meandering stream channel with the updip permeability barrier provided by a clay-filled, abandoned channel. This interpretation is applicable to other Fall River oil fields, as shown by distribution of sandstone at Coyote Creek and Miller Creek fields. On the basis f thickness of permeable sandstone, a pattern is present which is remarkably similar to that of point-bar, swale, and channel deposits of recent stream-meander belts. Recognition of these distinct facies will aid in exploration for new fields, in development drilling within fields, and in planning for secondary recovery projects.