Sally J. Sutton

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.

Stratigraphic Control of Chemistry and Mineralogy in Metamorphosed Witwatersrand Quartzites

Within metamorphosed quartzites of the Witwatersrand Supergroup bulk chemistry is the major control on the distribution of metamorphic mineral phases and on their chemistry. Bulk chemistry is in turn controlled by stratigraphic position. Pyrophyllite distribution varies directly with, whereas feldspar distribution varies inversely with, bulk Al. Chloritoid distribution is more complex and varies with both bulk Al and bulk Fe/Mg. Despite the overprint of low-grade metamorphism and, possibly, of other alteration processes, both detrital and metamorphic phyllosilicates can be recognized and characterized. Al in metamorphic white mica and metamorphic chlorite varies with bulk Al; whereas the composition of detrital white mica is unrelated to bulk chemistry. Similarly, Mg in metamorphic chlorite varies with bulk Mg. Bulk chemical variation, particularly in Al, Mg, and Na, and thus metamorphic mineral chemistry, is controlled by stratigraphic position, both within individual stratigraphie units and in the stratigraphie succession as a whole. K content increases upward and varies more strongly with position in the entire section than with position in individual units. The quartzites display a high degree of chemical maturity, as measured by the Chemical Index of Alteration, which increases upward both within individual stratigraphie units and in the section as a whole. The strong stratigraphie control on bulk chemistry suggests that intense weathering occurred in the source area and that alteration events associated with faults and other structural features play no more than a minor role in controlling variations in bulk chemistry. The observation that these rocks are strongly leached, yet retain detrital pyrite and uraninite, indicates that the source area weathering occurred under reducing conditions.

Deformation by Overburden of a Coarse Quartzite Conglomerate

The conglomerate of the Pass Peak Formation, a coarse Eocene basin-edge deposit in western Wyoming, is crushed though not by tectonic forces. Each cobble is in point to point contact with up to 10 other cobbles within a poorly consolidated micaceous sand matrix. Crushing appears to be due to stress concentration at contact points. Three degrees of deformation have been defined: (1) faint contact marks on the cobble surfaces where the surface polish has been destroyed and a rim of better lithified matrix formed; randomly oriented and distributed fractures cutting a few grains within the cobble; (2) throughgoing fractures, often parallel to one another with intervening complex networks of minor fractures, not necessarily starting or ending at a contact mark; (3) reduction of grain size by crushing up to 60% around contact marks; contact marks are enlarged and bundles of fractures form a complex network connecting these contact marks. Simple calculations show that the 700 m of overburden over the outcrops studied were sufficient to cause failure. A probable scenario is initial contact at minute contact marks which subsequently crush and enlarge as the overburden increases; the contact marks enlarge until the load is supported. No pressure solution was observed in these rocks. However, the crushing process within the Pass Peak may be a common precursor to deformation at higher temperature and pressure. This crushing has the effect of increasing the permeability and decreasing the grain size, especially in the vicinity of contact marks which may aid less brittle processes.

Development of Domainal Slaty Cleavage Fabric at Ocoee Gorge, Tennessee

Slaty cleavage developed in finely interlayered phyllite and metasiltstone of eastern Ocoee Gorge, Tennessee is defined by zones enriched in cleavage-parallel white mica (P domains), alternating with zones enriched in quartz and feldspar (Q domains). P domains are better developed in phyllite than in metasiltstone layers, where they tend to be discontinuous and widely spaced. Growth of new mica and recrystalliza-tion of clays and mica appear to have been essential processes in the development of the domainal slaty cleavage fabric. Constituent components of mica, in particular potassium, may have been transported into P domains, perhaps by fluids generated during diagenesis and low-grade metamorphism of these argillaceous sediments. Pressure solution of quartz and its mass transfer from P to Q domains may have contributed to the development of some domains but does not appear to have occurred universally. Similarly, a degree of passive concentration of resistant phases, such as rutile, probably occurred within some P domains and may have been due to pressure solution transfer to quartz. Evidence supporting these conclusions is textural and chemical and includes (1) the greater concentration of K than of Ti in most P domains relative to neighboring Q domains; (2) the greater mica/chlorite ratio in P than in Q domains; (3) compositional differences between P and Q domain phyllosilicates; (4) geometry of quartz overgrowths that suggest many quartz grain shapes are controlled by overgrowth precipitation, rather than pressure solution; and (5) P domain morphologies that suggest P domains propagated upward from phyllite layers into metasiltstone layers.

Orientation-Dependent "Metamorphic Grade" in Phyllosilicates Belonging to a Slaty Cleavage Fabric

The intimate coexistence of two microstructurally defined phyllosilicate populations that have apparently achieved two different degrees of diagenetic/metamorphic maturity underscores the importance of chemical processes in slaty cleavage development. Petrographic observations and analytical data on phyllites from Ocoee Gorge, Tennessee, indicate that most of the cleavage-parallel phyllosilicates concentrated in thin lamellae grew during low-grade metamorphism. On the other hand, the bedding-plane parallel phyllosilicates found between lamellae are detrital or diagenetically altered grains that were in place before slaty cleavage developed. Electron microprobe and XRD analyses have been used to characterize the mica, interstratified mica/chlorite, and chlorite found in the two microstructurally defined populations. Cleavage-parallel phyllosilicates are homogeneous and have crystal chemical characteristics indicative of low-grade metamorphism. Bedding-parallel phyllosilicates, on the other hand, are more heterogeneous and appear to be detrital or diagenetic. Specific differences in the two groups include greater K content of cleavage-parallel mica and the greater chemical variability of bedding-parallel mica. Cleavage-parallel interstratified mica/chlorite is limited to a maximum of about 50% chlorite, whereas there is no limit on chlorite in bedding-parallel interstratified grains. In addition, the proportion of mica in the total phyllosilicates is higher parallel to cleavage than parallel to bedding. The differences between the cleavage-parallel and bedding-parallel phyllosilicates suggest that new grain growth and recrystallization of pre-existing grains were more important than passive concentration and rotation in the development of this fabric.

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.