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uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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uFigure captions

uBasin systems

uSeals & compartments

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uBasin systems

uSeals & compartments

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Figure Captions

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Basin Hydraulic/Pressure Systems

 In most deep sedimentary basins in the world there is a layered arrangement of at least two superimposed hydraulic systems (Figures 1 and 2). The shallowest hydraulic system can extend to great depths; however in many basins it extends from the surface down to about 10,000 feet (greatest historical depth of burial) in normal geothermal gradient basins and to slightly greater depths in cool basins. There are a few remarkable deviations, like the central North Sea Basin, the South Papua Basin, the outer Gulf of Mexico and the Canadian Arctic Basin where the base of the shallow system has apparently never been buried more than about 4000 to 6000 feet.  

The shallow hydraulic systems are basinwide in extent and exhibit normal pressures. The pore water apparently is free to migrate; however, the usual rate of movement, below the uppermost few hundred feet, is so slow that motion is surmised rather than detected. Stable isotope ratios of dissolved solids and gases appear to indicate widespread invasion of the shallow hydraulic system by meteoric water in only a few basins.  

The deeper hydraulic systems usually are not basinwide in extent and exhibit abnormal pressures. They generally consist of a layer of individual fluid compartments which are sealed off from each other and from the overlying system. In some basins, mainly in the onshore U.S., there is an even deeper, near normally pressured noncompartmented section (Figures 3 and 4). The compartmented layer in those basins generally is in the sequence of rocks which were deposited during the period of most rapid deposition. The underlying noncompartmented layer, where present, usually is in pre-basin shelf deposits and basement rock. The uppermost noncompartmented layer usually is in rocks which were deposited during the slowing rate of deposition late stage in basin filling.  

The individual compartments in the compartmented layer are like huge bottles. Each one has a thin, essentially impermeable, outer seal and an internal volume which exhibits effective internal hydraulic communication. The rate of increase in pressure with increasing depth within the internal volume is in direct accordance with the density of the internal fluids (Figure 5). The fluid pressures in the internal volume may be greater than, equal to, or less than the pressures in the fluids in the rocks outside of the seal. The magnitude of the internal fluid pressure is dependent on how much of the weight of the superincumbent rock column is borne by the fluids in the enclosed body and how much of the weight is borne by the rock matrix in the enclosed body. The fluid pressure below the top seal at the shallowest point in the enclosed rock body can range from zero, where the rock matrix bears all of the weight of the superincumbent rock, to about 1 psi/foot thickness of overlying rock if the enclosed rock matrix bears none of the weight of the superincumbent rock and water load.   

The individual compartments in the compartmented layer may be very extensive, as in some of the Rocky Mountains basins, or may be only a few miles across, as in the Gulf Coast Basin. The pressures within the compartments generally are overpressured or underpressured relative to the pressures in both the shallower and deeper hydraulic systems (Figure 6). The compartmented hydraulic systems in currently sinking basins are almost universally overpressured and are underpressured in many onshore basins undergoing erosion. The principal sources of overpressures appear to be thermal expansion of confined fluids and the generation of petroleum during continued sinking, and the principal source of underpressures appears to be thermal contraction of confined fluids as buried rocks cool during continued uplift and erosion at the surface. Thus, it appears that the compartments have an amazing longevity as they undergo a continuum from overpressures through normal appearing pressures to underpressures as their host basins progress from deposition, to quiescence, to basin uplift and erosion.  

In those basins with three layers of hydraulic systems, the seal between the middle compartmented layer and the underlying noncompartmented layer usually follows a single stratigraphic horizon. For instance, the basal seal of the compartmented section in the central Powder River Basin appears everywhere to be within the thin Cretaceous Fuson shale. However, in many basins, the top seal of the compartmented layer is more complicated. (1) It tends to follow an irregular sands-over-massive-shale boundary in the Gulf Coast and Niger Delta basins; (2) it is within thin evaporites in many onshore European and southwestern U.S. basins; and (3) it occurs as horizontal or gently dipping planes which cut indiscriminately across structures, facies, formations, and geological time horizons in the Alaska North Slope Basin, in the northern Cook Inlet Basin, in the Alberta Basin, in the Anadarko Basin, in the North Sea Basin, and in many Rocky Mountains basins (Figure 3). Those top seals which do not follow a specific stratigraphic horizon generally are restricted to clastics-dominated sections. Planar seals may occur on the top, bottom, or within compartments.  

The planar-topped, compartmented sections are almost universally in basins which are older than the basins in which the compartmented sections exhibit much top surface irregularity. Thus, it appears that there is some process in nature whereby the top seals of compartments in clastics-dominated sections can smooth themselves over time. The leveling process may be quite rapid because the tops of the two principal fluid compartments in the central North Sea Basin are horizontal over distances in excess of 100 miles despite the recent salt-induced structure development in the area.

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Seals and Compartments 

Recognition of the layered arrangement of hydraulic systems generally is quite easy. Only a few widely spaced, well-documented deep wells with several pressure tests run over perforated intervals or several pressure readings from repeat formation testers in scattered wells generally are sufficient to outline the overall arrangement of hydraulic systems in each basin. Pressure/depth profiles are remarkably similar in most deep basins in the world (Figures 7, 8, 9, 10, 11, & 12). The similarity suggests that the formation of seal-bounded fluid compartments is part of normal basin development.  

Seals are particularly annoying to work with because they do not have consistent lithologic properties other than extremely low across-the-seal permeability. In the absence of unique lithologic properties, recognition must be accomplished from indirect evidence, such as well log indicators, measured pressures in local reservoirs encased in seal rock, and often only from the requirement that they must be there separating reservoirs which, from measured pressure data, are obviously hydraulically separated from each other. Seals may have thin internal permeable rock layers (like bubbles in the glass of glass bottles), which contain water or oil and gas pools. The transition of pressures across the total thickness of top seals in clastic rocks is linear with increasing depth wherever data have been obtained (Figure 13). Too few data have been accumulated to determine the patterns of pressures within lateral seals or within basal seals. The overall rate of pressure change across seals in sha1e has been observed to be as great as 15 psi/ft and 25 psi/ft in seals in sandstone.  

In some areas, seals may be recognized by calcite and/or silica mineralization within the seals or in the lower pressured rocks exterior to the seals, probably resultant from dissolved minerals being precipitated as water seeps through the seals. The mineral infi1l of porosity and fractures may be so readily recognizable that it becomes an identifier of present or past seals. For instance, calcite infill is so ubiquitous within seals and in adjacent beds in southwestern Louisiana that it has been given the name “Al's Cap,” named for Al Boatman, a local geologist, who first publicly drew attention to the phenomenon there. Silica infill may be recognizable on the basis of drastically reduced rates of drilling penetration across a seal. For instance, it took 24 hours to cut a 60-ft core in a silica-enriched seal in chalk in the Shell-Esso 30/6-2 well in the North Sea. Chalk normally cores very rapidly, unless the bit becomes clogged. Several well log interpretation techniques have been developed to recognize the changes in pressures across seals and to recognize the mineralized rocks associated with seals (Figure 14). 

Top seals in clastics-dominated sections range in thickness from 150 feet to over 3000 feet; however, the majority are uniformly near 600 feet. Seals in carbonate-evaporite sections are generally somewhat thinner; in fact, some salt and anhydrite beds as thin as 10 feet form effective seals. An example of the latter is the Devonian Davidson evaporite, which, except for a small area in central Saskatchewan, is about 20 feet thick but forms a regional seal over almost the entire extent of the Williston Basin.  

Lateral seals appear to be generally vertical or very nearly vertical. They range in width from less than 1/8 of a mile (within the distance between wells on 10-acre spacing) to about six miles, with the majority being 1/8 of a mile or less in width. They tend to be quite straight, which suggests that they may tend to follow fault trends. There has not been any satisfactory suggested geochemical mechanisms which could create impermeable walls over thousands of feet of vertical extent through rocks of many lithologies. Where wells have penetrated lateral seals, the rocks have generally been found to be slightly fractured and the fractures infilled with calcite and/or silica. In a few localities, some of the fractures are locally open and can yield limited oil and gas production. While lateral seals are almost always nearly vertical, continuous planes, there are a few remarkable cases of breaks in seal continuity where individual permeable rock layers extend in hydraulic continuity from a compartment into a neighboring compartment. Those tongues are of particular interest to exploration geologists because they frequently contain oil and gas pools.  

The rocks in the internal volumes within the compartments, like the seals, do not have a unique lithology. The most unique property is the pervasiveness of fractures observed in cores and indirectly indicated by the apparent hydraulic continuity (i.e., reservoir to reservoir continuity of interval pressure-depth profiles) within the internal volumes. A few authors, most notably Narr and Currie (1982), have attempted to explain a genetic mechanism for the fractures; however, none of the explanations to date have been particularly convincing. The fractures in underpressured through slightly overpressured Cretaceous and older rocks are generally nearly closed in most basins; however, they are generally open enough to cause prominent reductions in overall interval sonic velocities in overpressured rocks. The fractures are open enough to take large quantities of drilling mud if the mud columns in drilling wells are slightly overbalanced in underpressured fluid compartments in the Hanna Basin and in the deep basin area of the Alberta Basin. Mud losses start at the base of the top seals in both areas. The mud will reenter the wellbores if the wells are changed to an underbalanced state. Most fractures are less than 1 inch long. They generally extend from pore to pore and tend to separate grains rather than break across grains.  

The fractures in the internal volume are, in a few areas, open enough to permit commercial-rate extraction of oil and gas even in the absence of significant matrix porosity and permeability. However, the distribution of open fractures is generally not uniform enough to allow field development without a substantial proportion of dry holes unless the fracture porosity is augmented with matrix porosity and permeability within the internal volume rocks. The matrix rocks, in different areas, may exhibit remarkably different porosity values. For instance, sandstone porosities are in the 20-35% range in the overpressured Cretaceous Tuscaloosa sandstone reservoir in the False River Field in Louisiana and are generally much less than 10% in the Paleozoic Goddard sandstone reservoir in the Fletcher Field in Oklahoma at approximately the same depth and pressure. 

Fluid compartments are important in subsurface geology because oil and gas is trapped in permeable beds where they abut seals, it is trapped within permeable beds within seals, or, in a few cases, compartments and their seals are completely filled with oil or gas. Fluid compartments apparently trap oil and gas for very long periods of time and may be important, from a national resource standpoint, in retaining petroleum at depths beyond the usual depth range explored to date. Underpressured fluid compartments probably will become important as sites for disposal of gas and liquid wastes.  

It would be highly desirable to better understand the subsurface environment in which fluid compartments are formed and continue to survive. The purpose of the talk today is to show sufficient hard data on fluid compartments in several basins around the world to allow the audience to acquire a balanced “feel” for the phenomena observed.  

Reference 

Stanescu, V., C. Carraru, and D. Varvarici, 1969, Abnormal pressure and structure of the gas bearing reservoirs of some salt domes of the Transylvanian Depression: Bulletin of the Institute of Petroleum, Geological Gazette, Bucharest, Romania, v. 17, p. 239-257

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