William R. Almon

Textural and sequence-stratigraphic controls on sealing capacity of Lower and Upper Cretaceous shales, Denver basin, Colorado

Shale units can be important barriers to fluid flow in sedimentary basins and commonly serve as seals to petroleum reservoirs. Little is known, however, about the controls on shale permeability. Consequently, variation in seal competency is one of the greatest risk factors associated with petroleum exploration.Here, we examine possible controls on sealing capacity in two Cretaceous marine shale units in the Denver basin, Colorado. Sealing capacity, as determined by mercury injection–capillary pressure analysis, is compared to several textural and compositional parameters and to sequence-stratigraphic setting. These two shale units display highly variable sealing capacity, even between some adjacent samples. This suggests that variability in some small-scale shale characteristics may strongly influence sealing capacity. The best seals are generally in transgressive systems tracts, especially within or immediately below condensed sections.Textural characteristics of shale appear to be especially important in determining sealing capacity. In particular, well-sorted pore-throat sizes and well-developed bedding-parallel preferred orientation of flattened organic matter particles strongly favor high sealing capacity. High degrees of bioturbation degrade sealing capacity, possibly by disrupting preferred orientation and by increasing variability in grain size and hence in pore-throat sorting. Preferred orientation of matrix clays parallel to bedding also appears to increase with increasing sealing capacity, but is probably less important than the preferred orientation of organic matter.Compositional characteristics are generally less important than textural characteristics in determining sealing capacity in these shale units. Neither silt content nor cement content appears to be important to sealing capacity in these shale units. Total organic carbon is generally high in samples with good sealing capacity, but can be either high or low where sealing capacity is poor.Overall, the variables that most strongly favor high sealing capacity, pore-throat sorting, organic matter bedding-parallel preferred orientation, and low bioturbation, are most likely in anoxic, deep-water settings, hence, the association between good seals and condensed sections.

Sedimentology and Petrophysical Character of Cretaceous Marine Shale Sequences in Foreland Basins—Potential Seismic Response Issues

Development of predictive models to estimate the distribution and petrophysical properties of potential mudstone-flow barriers can reduce risks inherent to exploration and exploitation programs. Such a predictive model, founded in sequence stratigraphy, requires calibration with outcrop and subsurface analogs. Detailed sedimentological, petrophysical, and geochemical analyses of Lewis Shale (lower Maastrichtian) samples from southeast Wyoming reveal considerable variability in petrophysically and seismically significant rock properties. Lower Lewis strata represent late-stage transgressive deposits that include a distinctive condensed interval. The overlying progradational Lewis interval consists mostly of interstratified very silty shales and argillaceous siltstones. High-frequency sheet and lenticular sandstone bodies occur in the progradational Lewis package. Sealing capacity, as measured by mercury injection-capillary pressure (MICP) analysis, varies with fabric, texture, and compositional factors that are related to sequence-stratigraphic position. Samples from the Lewis Shale transgressive interval have significantly greater MICP values (average 18,000 psia) and are markedly better seals relative to samples from the overlying Lewis Shale progradational package (average 3000 psia). Transgressive shales with enhanced sealing capacity are characterized by higher total organic carbon and hydrogen index values, lower permeability, and lower detrital silt content. These transgressive shales are enriched in iron-bearing clay minerals and authigenic pyrite. Greater shear wave velocities, larger shear moduli, and higher bulk density also characterize transgressive Lewis Shales. The most promising seal horizons are laterally extensive, silt-poor, pyritic shales occurring in the uppermost transgressive systems tract. Stacking patterns of slow and fast shale horizons can yield seismic responses comparable to those interpreted as hydrocarbon-bearing reservoirs.

Development of Diagenetic Seals in Carbonates and Sandstones: ABSTRACT

Diagenetic seals effectively block the movement of reservoir hydrocarbons in many sandstone and carbonate rock units. Diagenetic seals in sandstones and carbonate rocks encase reservoir rocks with either depositional or diagenetic porosity. Diagenetic reservoir porosity may originate before or after the establishment of an effective diagenetic seal. Hydrocarbon traps with diagenetic seals may conform in their geometry as well to structure or stratigraphy as to diagenetic facies. Therefore, some structural and stratigraphic traps may, in part or entirely, depend on diagenetic seals. Detailed analysis of diagenetic seals in sandstones and carbonate rocks can considerably improve our ability to predict their occurrence and to recognize their spatial and temporal relationship to reservoir rocks and hydrocarbon migration.

Textural Controls on Sandstone Diagenesis: ABSTRACT

Diagenetic alterations of sandstone occur in a continuous system. As a result, equilibrium thermodynamics cannot be strictly used to describe the equilibrium composition of the diagenetic system and the resulting course of diagenesis. If a geologist is to predict the course of diagenesis in a meaningful way, he must determine those factors which serve to control the various diagenetic pathways. Geologic evaluation of sandstone fabric and texture is an integral part of most regional studies. These data are often critical in understanding diagenesis as well. Sediment grain size, roundness, sorting, and packing factors determine the ability of a sandstone to transmit fluid during the course of burial and diagenesis. These geologic factors can be used to evaluate the paleohydrology End_Page 411------------------------------ of the sand bodies under investigation, and provide insight to a major control of diagenesis. The fluid flow rate through a sandstone controls the residence time of the various chemical components in solution. When the residence time is sufficiently long with respect to the time scale of the diagenetic reaction, the time invariant condition of a continuous system approaches chemical equilibrium. Thus it is possible to have different diagenetic reactions occurring within a sand body due to local changes in fluid flux (or flow velocity) and the resultant varying degrees of approach to equilibrium. In systems where fluid flow is high and residence time is small with respect to diagenetic reaction rate, the fluid chemistry is largely controlled by the chemistry of external fluid source. This situation results in either introduction of new material (or minerals) into, or removal of existing material or minerals from the sandstone. If the fluid is saturated or nearly saturated with respect to some specific mineral this mineral is added. Conversely, leaching of material occurs when the fluid is undersaturated. In systems where fluid flow is slow and residence time is large with respect to diagenetic reaction rates, the fluid chemistry is largely controlled by the chemistry of the host rock. In this case, chemical equilibrium is approximated and the material originally present in the rock is redistributed by solution/precipitation reactions. Only small amounts of materials are introduced to or removed from the host rock. Understanding this control on sandstone diagenesis is important in delineating trends in diagenetic alteration and projecting those trends into new areas, and in identifying the trends in differential cementation that produces some types of diagenetic traps. End_of_Article - Last_Page 412------------

The Role Of Diagenesis In Successful Formation Interpretation, Well Production, Stimulation, And Enhanced Recovery

Post depositional modifications (diagenesis) of sandstones are described to play an important role in controlling hydrocarbon discovery and production. Diagenesis can affect the ways in which geologists and engineers interpret, evaluate and develop sandstone reservoirs.

Impact of Diagenesis on Log Interpretation: ABSTRACT

Significant problems arise in the interpretation of wire-line logs from diagenetically altered rocks. How does one interpret a zone that on the logs appears water filled or noncommercial? Is the zone truly water filled? If the zone is water filled, does it serve as a seal for a downdip hydrocarbon accumulation? An understanding of the degree of diagenesis and its timing will allow the wire-line logs to be interpreted in the proper manner and will prevent the abandoning of potentially productive wells and lead to the discovery of hydrocarbons held in diagenetic traps. End_of_Article - Last_Page 1595------------

Mechanisms of Formation Damage in Diagenetically Altered Sandstones: ABSTRACT

Diagenetically altered sandstones (shaly sands) are extremely susceptible to formation damage during drilling, completion, stimulation or enhanced-recovery operations. This susceptibility arises because of the development of diagenetic minerals within the pore system and because of physical and chemical interactions between the diagenetic minerals and the injected fluids. Formation-damage mechanisms can be separated into 3 groups: mechanical, chemical, and physical. The mechanical mechanisms include migrating clays, swelling clays, and sloughing clays. The chemical mechanisms include dissolution of minerals and precipitation of minerals. Physical mechanisms of formation include emulsion blockage or water blockages. The scanning electron microscope allows the geologist or engineer to observe directly the mechanisms for formation damage in a reservoir and to design his treatment accordingly. The SEM also can be used to observe the effects of well treatment and evaluate their success or failure.

Abstract: Effects of Sandstone Composition and Diagenesis on Reservoir Quality, Tertiary-Pleistocene, Gulf Coast Region

ABSTRACT The quality of sandstone reservoirs in the Tertiary-Pleistocene sequence of the Gulf Coast is affected by both original composition and diagenesis. Porosity and permeability are controlled by 1) original depositional matrix, 2) grain crushing, 3) solution and reprecipitation of carbonate shells, 4) authigenic clay pore-linings, 5) authigenic clay and carbonate pore-fills, and 6) development of secondary porosity. Factors 1 through 4 act as important controls on reservoir quality in Pliocene-Pleistocene offshore reservoirs. All six factors combine to control reservoir quality in pre-Pliocene sandstones. The presence of diagenetic minerals in a sandstone can totally aIter the electric log response. A log may indicate that a sandstone is water saturated, and yet the well may produce water-free oil. This is commonly a result of the presence of authigenic clay pore-linings which bind water to the framework grains. Hydrocarbons may thus be seriously underestimated when a sandstone contains only a few percent authigenic clays. The acid or fresh water sensitivity of a sand may also be controlled by the diagenetic minerals. Diagenetic pore-linings receive the greatest exposure to drilling, treatment, and completion fluids, and greatly effect reservoir quality. A knowledge of both sandstone composition and diagenesis are essential in defining optimum exploration strategies, as well as drilling and stimulation treatments. End_of_Record - Last_Page 244-------