Table of contents | Glossary | Geology | Depositional model | Heterogeneity |Diagenetic history| Petroleum geology | 1) East-West porosity slices | 2) North-South porosity slices | 3) Onion views of 3-D images | 4) Horizontal slices of porosity | 5) Horizontal and East-West permeability and porosity slices

Diagenetic History of the Sussex "B" Sandstone

The Sussex "B" sandstone exhibits a complex diagenetic history that strongly influenced porosity and permeability preservation, loss, and enhancement. These factors in turn affect emplacement, trapping, and production of petroleum. The Sussex "B" sandstone is a litharenite to feldspathic litharenite, based on the classification of Folk (1970). The early, middle, and late diagenetic stages are shown in the following figure 24. High initial rates of compaction occur mainly in muds, and in sands that were not lithified, or supported, by early quartz and (or) calcite cementation. Rates of compaction decrease with time. The dashed lines are time periods of dissolution of grains and cements, which develops secondary porosity and permeability. Diagenetic stages were derived from examining the following; 1) cores of 10 wells, 2) 54 thin sections (including cathodoluminescent thin sections), 3) numerous SEM images, and 4) X-ray fluorescence (XRF) and diffraction (XRD) charts.

Figure 24. Generalized paragenetic sequence of the Sussex "B" sandstone (diagenes.gif). Dashed lines indicate times of dissolution of cements or grains. Yellow zone is period of sediment compaction. Medium blue zone is period of migration of hydrocarbons from shale source rocks. Dark blue zone is time of oil emplacement into the House Creek and Porcupine fields.

Very Early Diagenetic Stages

Precipitation of chlorite, pyrite, and alteration of fecal pellets to glauconite results from reducing conditions in marine pore fluids; this occurs soon after burial. Accumulation of bacterial end-products in pore fluids often results in precipitation of carbonate minerals and iron sulfides (Clayton, 1992). These end products affect pore water pH, Eh, and ionic strength.

The earliest diagenetic event is compaction. This is most significant for sediments with a high mud content; sand-sized grains floating in the mudstone matrix exhibit few and minor diagenetic changes. This results from isolation of these grains from pore fluids that affected cementation and dissolution. Rate of compaction of sediments decreases through time; later diagenetic cementation of sandstones by calcite and quartz stabilizes the grain framework; this decreases porosity and permeability loss through rearrangement of grains.

Even though three periods of calcite cementation are shown on figure 24, there is actually an additional stage of very early diagenetic calcite cementation. Because it is very limited in scope and distribution, it was not added to the figure. The following transmitted (rfca1sm.gif) and plane-polarized (rfcasm.gif) light images are radial-fibrous ferroan calcite. Radial-fibrous and planar-laminar calcite precipitates rapidly from pore fluids that are highly supersaturated in the carbonate ion. Bounding quartz grains exhibit peripheral dissolution but do not contain the later diagenetic chlorite rims or quartz overgrowths.

Figure 25. A. Transmitted-light view of very early diagenetic radial-fibrous calcite (C) that displaced quartz (Q) grains and glauconite. Calcite precipitated within the glauconite pellet and blasted it apart. Grains float in the matrix of calcite cement and mudstone (MS). Rfca1.gif is the full 752 KB transmitted-light image. B. The star-pattern of the calcite is apparent on this plane-polarized view. Rfca.gif is the full 736 KB plane-polarized image. Blue color results from staining the thin section for ferroan calcite.

An early-diagenetic stage of grain dissolution and replacement by calcite cement (alteration of lithics on figure 24) is indicated by standard and cathodoluminescent (CL) examination of several thin sections. The Figure 26 (clumcmts.gif) photomicroscope image of a chert-pebble lag shows the following: 1) Chert, quartz, and feldspar float in a matrix of ferroan calcite (orange) cement, 2) the "bright" spots or ghosts on the sample are not grains; they are calcite cement that is optically continuous with surrounding calcite cement. These are called ghosts because they show locations of grains that were replaced by the calcite. Ghosts are invisible using standard petrographic methods, except when floating grains and fragments suggest their origin. Because they are invisible, calculated minus-cement porosity volumes would be too great for these samples (sediment porosity near the time of deposition). Visual estimation of volume of grains and fragments removed during this stage is 1 to 5 percent.

Figure 26. Cathodoluminescent photomicrograph of ferroan calcite cement (orange and bright orange) within a chert-pebble lag. Calcite is optically continuous across this view. Grains of chert (CT), quartz (Q), and feldspar (striped grain) float in a matrix of calcite cement. The feldspar grain is partially replaced by this very early diagenetic calcite. Bimodal grain distribution and point contacts between grains are also shown. clumcmt.gif is the full-scale 736 KB image. Quartz overgrowths are black (visible on the full-scale image). Scale bar is 0.1 mm.

Bimodal grain distribution results from erosion of underlying ridge sediments and incorporation of grains into the chert lag. Quartz grains exhibit very minor (meniscal in appearance) quartz cementation, that in turn has peripheral dissolution by the early diagenetic calcite cement. Quartz grains commonly have coatings of early-diagenetic chamosite. The style of early diagenesis represented in this lag and the underlying sandstone is characteristic for marine pore fluids; the sediments were probably not exposed to fluctuating ground water in a vadose zone. The meniscal (concavo-convex) occurrence of authigenic quartz results mainly from peripheral dissolution of the poorly developed overgrowths.

Early to Middle Diagenetic Stages

Rims of chlorite grow perpendicular to quartz grain surfaces and appear to be intergrown with the quartz cement, as shown on the figure 27 SEM image (semchlsm.gif). Quartz displays well-developed overgrowths. X-ray diffraction analysis (Higley, 1994) indicates that most of the Sussex "B" chlorite is ferroan chlorite, or chamosite. Fuchtbauer (1983) postulates that chlorite, chamosite, and glauconite are found almost exclusively in marine environments.

Figure 27. Scanning electron microscope (SEM) images of early-diagenetic chlorite rims (bladed crystals) that coat quartz grains and are intergrown with quartz overgrowths, No. 14-9 Federal well. Brown box shows location of the zoomed-in image. A pore space is labeled (ellipse with a line through it). Semchl.gif is the full-scale 768 KB image.

Precipitation of (middle diagenesis) calcite cement prior to cementation by quartz is illustrated by the figure 28 cross-plot of depth in feet from surface versus volumes of a) calcite cement, b) quartz cement, and c) core porosity and d) permeability. Sussex "B" sandstone average porosity and permeablity of 13 percent and 15 mD are shown, as well as Sussex core and Shannon outcrop exposures that approximate core facies described in Higley (1988, 1994). Porosity and permeability exhibit general upward increase, which correlates with increase in depositional energy (trough-cross-bedded sandstone), and decrease in amounts of calcite cements. Twin spikes of about 13 percent quartz cement are mirrored by low percentage of calcite cement.

Figure 28. Shown are 1) 20 to 0 percent calcite (CO3++) and 14 to 4 percent quartz (QTZ) cements, 2) 20 to 0 percent core porosity (POR), 3) 100 to 0.01 mD permeability (K), and 4) photographs showing facies for the Sussex "B" sandstone, Empire Federal "C" oil well. TRSS is trough-cross-bedded sandstone intervals; these are concentrated near the top of the "B" sandstone and are the primary reservoir facies. "THIN-BEDDED VF-F SS, MS LAMINAE" refers to the thin-bedded horizontally stratified fine-grained sandstone and interbedded mudstone. Bioturbated mudstone is the blue-gray-colored rock located mainly at the base and top of this section. Heavy vertical bar on the right side of the figure is the perforated zone. The full-scale 144 KB image is empfedsr.gif.

Early diagenetic alteration of the hash is indicated by presence of the competent siderite clasts on Sussex "B" trough-cross-bedded sandstones (figure 29). The initial stage of calcite cementation in the Sussex "B" sandstone is probably contemporaneous with alteration to siderite of foraminiferal tests and other depositional carbonate hash (figure 30).

Figure 29. Core photograph (trs8167s.gif) from 8,167-ft depth in the Mandel Federal "C" well shows siderite clasts that were ripped up by high-energy currents and redeposited on the trough-cross-bedded sands of submarine dunes. Original horizontal bedding is shown in clasts by the stringers of quartz grains. The full-scale trs8167.gif image is 992 KB.

Figure 30. Thin-section photomicrograph of a siderite clast in a chert-pebble lag, No. 14-1 Federal well, Porcupine field (foramsm.gif). Foraminifera tests in the clast have been partially replaced by siderite (brown). Evidence for the marine origin of the siderite clasts includes alteration to siderite of Haplophragmoides and Reophax foraminiferal tests. The full-size foram.gif image is 768 KB.

Middle to Late Diagenetic Stages

A number of diagenetic stages are shown on the figure 31A, B transmitted-light and crossed-nicols photomicrographs (pyrca1sm.gif, pyrca2sm.gif). Pyrite (P) framboids (similar in appearance to raspberries) are black in transmitted-light and crossed-nicols views. These late-diagenetic pyrite crystals are commonly associated with the dolomite and may result from changes in chemical composition of pore fluids resulting from migration of hydrocarbons into the Sussex "B" sandstone.

Figure 31. A. Transmitted-light and B. crossed-nicols thin-section photomicrographs of pyrite framboids (P) within ferroan dolomite (D) from a trough-cross-bedded sandstone at 8,014-ft (2,443 m) depth in the Empire Federal "C" oil well. Quartz (Q) and feldspar (F) grains bound optically-continuous calcite (C) cement. G is a glauconite pellet. Medium blue is epoxy and mottled blue results from kaolinite clay (transmitted light view), dark blue on both views is the late-diagenetic ferroan dolomite. Full-size transmitted-light image is 752 KB pyrca1.gif. Full-size crossed-nicols view is 784 KB pyrca2.gif.

Quartz and feldspar grains in the figure 31 thin section display well-developed overgrowths. Overgrowth formation was followed by precipitation of optically continuous, non-ferroan, poikilotopic, zoned calcite cement (C). Figure 31B reveals subsequent spotty dissolution of the calcite cement by distribution of optically continuous fragments. This was followed by filling of the secondary pore by ferroan dolomite and pyrite. Also visible are a glauconite pellet (G) that has been slightly altered to illite (bright speckles under crossed nicols), and plagioclase feldspar (F, left border of images) that underwent extensive late-diagenetic dissolution and recrystallization. Original grain outline is indicated by brown dead oil (latest diagenetic emplacement).

Dissolution of the ferroan calcite cements resulted mainly from changes in pore fluid chemistry, primarily organic and carbonic acids associated with hydrocarbon generation and migration into the reservoir. This created secondary porosity, and provided the primary source of chemically reduced iron for pyrite and ferroan dolomite. Migration also resulted in dissolution of feldspar and subsequent precipitation of kaolinite in pores.

Late-stage influx of oil into the field area is indicated by both petrologic analysis of thin sections and burial history reconstructions. Thin sections that are oil saturated generally exhibit well-developed quartz overgrowths, dissolution of unstable lithics, and other stages characteristic of middle to late diagenesis. The time of oil generation for source rocks of the Sussex "B" sandstone is within the time range of 40 Ma to possibly 10 Ma (Higley, 1994). Cody Shale samples from six wells in and near the House Creek field are marginally mature for oil generation and migration; mean random vitrinite reflectance (Rm) for nine samples ranges from 0.51 to 0.56 percent Rm at current depths of 7,700 to 9,800 ft (2,300 to 3,000 m). This Rm range is lower than the 0.6 percent to 0.65 percent Rm onset of oil generation for types II and III kerogen (Waples, 1985) that characterize these marine shales. Absence (currently) of faulting, and encasement of the reservoir in low-permeability shales of the Cody Shale suggest that the oil was generated in place, or at least did not experience long-distance migration. Suppression of vitrinite is also a possibility, with this scenario the Rm values would be slightly greater than recorded. A possible source of oil within the House Creek field is more thermally mature Cody Shale that is located deeper in the basin (Higley, 1994)

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