Nicholas B. Harris

Mechanical anisotropy in the Woodford Shale, Permian Basin Origin, magnitude, and scale

The rock mechanical properties of shales are critical to how they perform as reservoirs, determining both their tendency to develop natural fractures and their response to hydraulic stimulation. These mechanical properties are, in turn, manifestations of composition, fabric and porosity, and fluid saturations. Such properties also dictate the seismic response of shales, as well as their signature on many well logs. It has long been recognized, or perhaps assumed, that shales are relatively homogeneous in the plane parallel to bedding and relatively heterogeneous in the direction perpendicular to bedding.

A sequence-stratigraphic framework for the Upper Devonian Woodford Shale, Permian Basin, west Texas

Criteria for recognizing stratigraphic sequences are well established on continental margins but more challenging to apply in basinal settings. We report an investigation of the Upper Devonian Woodford Shale, Permian Basin, west Texas based on a set of four long cores, identifying sea level cycles and stratigraphic sequences in an organic-rich shale. The Woodford Shale is dominated by organic-rich mudstone, sharply overlain by a bioturbated organic-poor mudstone that is consistent with a second-order eustatic sea level fall. Interbedded with the organic-rich mudstone are carbonate beds, chert beds, and radiolarian laminae, all interpreted as sediment gravity-flow deposits. Bundles of interbedded mudstone and carbonate beds alternate with intervals of organic-rich mudstone and thin radiolaria-rich laminae, defining a 5–10 m (16–33 ft)-thick third-order cyclicity. The former are interpreted to represent highstand systems tracts, whereas the latter are interpreted as representing falling stage, lowstand, and transgressive systems tracts. Carbonate beds predominate in the lower Woodford section, associated with highstand shedding at a second-order scale; chert beds predominate in the upper Woodford section, responding to the second-order lowstand. Additional variability is introduced by geographic position. Wells nearest the western margin of the basin have the greatest concentration of carbonate beds caused by proximity to a carbonate platform. A well near the southern margin has the greatest concentration of chert beds, resulting from shedding of biogenic silica from a southern source. A well in the basin center has little chert and carbonate; here, third-order sea level cycles were primarily reflected in the stratigraphic distribution of radiolarian-rich laminae.

The impact of rock composition on geomechanical properties of a shale formation: Middle and Upper Devonian Horn River Group shale, Northeast British Columbia, Canada

The geomechanical properties of a shale reservoir are essential both to the development of natural fractures and to the formation’s response to hydraulic fracture stimulation. We evaluate the rock mechanical properties in the Middle and Upper Devonian Horn River Group shale, including the Evie and Otter Park Members and the Muskwa Formation, applying core hardness measurements and log-derived Youngs modulus, Poissons ratio and brittleness and relate these to shale composition and texture. Clay content is the most significant factor controlling the brittleness of shale rocks. The effect of quartz content on rock mechanical properties depends on the type of quartz present in the rock: authigenic quartz is positively correlated with brittleness, but detrital quartz has little or no effect. Factor analysis indicates that carbonate increases brittleness, while no obvious correlation between TOC content and brittleness was observed. Depositional facies are related to geomechanical properties because each facies has a distinctive composition. Massive mudstone and pyritic mudstone are relatively brittle because of abundant authigenic quartz cement. Laminated mudstone and bioturbated mudstone are relatively ductile because most detrital carbonate and quartz grains are set in a matrix of clay minerals. Brittleness in the Horn River Group shale shows both geographic and stratigraphic variability. Increasing brittleness in the northwest part of the basin largely results from greater distance from the clastic sediment source. The Otter Park Member represents a period of major relative sea level fall and is relatively ductile because of its high clay content, whereas the underlying Evie Member and the overlying Muskwa Formation are relatively brittle because of high carbonate and biogenic silica content, respectively.

ABSTRACT: Distribution, Timing And Intensity of Major Tectonic Events on The West African Margin from Gabon to Namibia: Results of A Regional Apatite Fission Track Study

The structure and morphology of the West Africa passive margin are thought to have been largely determined by Early Cretaceous rifting and formation of the South Atlantic Ocean, although researchers have locally identified Tertiary and Late Cretaceous uplift events, some of which are clearly associated with unconformities. Assumption of a passive margin model have guided exploration assessment of reservoir, seal and source rock potential.

ABSTRACT: Organic Carbon-Sulfur Relations in Lacustrine Shales of the West African Basins: Implications for Paleoclimate and Source Rock Potential

The chemical composition of Early Cretaceous syn-rift shales from the Gabon, Congo and Kwanza basins has been analyzed for sulfur and total organic carbon (TOC). Organic carbon sulfur trends in the three basins define very different trends: 1) Gabon low sulfur and low correlation with %TOC and a possible slight decrease in sulfur with %TOC; 2) Congo low to moderate sulfur, strong correlation with %TOC and moderate increase in sulfur with %TOC ; 3) Kwanza moderate to high sulfur, strong correlation %TOC and strong increase in sulfur with %TOC. The sulfur organic carbon trends coincide with clear differences in sedimentary lithology and structures. The Kwanza sedimentary section shows clear indications of aridity, including common carbonate beds, red beds, shallow water lakes and evaporites. The Congo section generally shows restricted carbonate beds, deep to moderate depth lakes, rare evaporites and no red beds. The Gabon section shows no carbonate beds, deep lakes and no evaporites or red beds.

Episodic hydrocarbon migration during deposition of the Frio clastic wedge, Gulf of Mexico: Its expression and causes

A petrographic and geochemical studies of the Oligocene Frio Formation show that the carbon isotopic composition of calcite varies systematically with stratigraphy, due to varying hydrocarbon migration to the sea bed during Frio deposition. In the St. Charles Ranch fields, Aransas County, Texas, compositions in the Lower Frio are relatively heavy ([delta][sup 13]C = [approximately]-2[per thousand]), decreasing stratigraphically upward to -5.5[per thousand] in the Middle Frio, then increasing to -2[per thousand] in the Upper Frio. The average calcite composition is identical in sandstones and mudstones at specific stratigraphic levels. This pattern contrasts with [delta][sup 18]O, in which sandstone and mudstone calcite has different compositions and no stratigraphic control is evident. Two limestone beds have very light [delta][sup 13]C compositions ([approximately] -20[per thousand]); they are analogous to modern carbonate buildups in the Gulf of Mexico forming around methane vents. The carbon isotopic composition of calcite reflects the flux of hydrocarbons to the sea bed: low [delta][sup 13]C values indicate greater incorporation of hydrocarbon into calcite and therefore a large hydrocarbon flux, and vice versa. The data suggest that a pulse of hydrocarbon migration occurred during Frio deposition, peaking during Middle Frio deposition. Two mechanisms for this migration pulse are considered: first,morexa0» deposition of the Frio resulted in sudden hydrocarbon maturation and generation in underlying source rocks; and second, growth faulting, associated with deposition of the Frio wedge, opened migration pathways. As sedimentation rates slowed during deposition of the Upper Frio, movement on growth faults decreased and migration pathways closed. These mechanisms are tested with numerical models.«xa0less

Diagenetic Pathways for Sandstones: The Role of Initial Composition: ABSTRACT

The initial composition of a clastic section is critical in determining the diagenetic reactions that a sandstone will undergo during burial, reactions which strongly influence its reservoir properties. The role of initial composition is illustrated for Middle Jurassic sandstones of northwest Europe (including the Brent sandstone of the North Sea) and Tertiary sandstones of the Gulf of Mexico. The composition of the former evolves from arkose to quartz arenite, with massive dissolution first of plagioclase and subsequently K-feldspar. As the bulk composition changes, the suite of clay minerals changes from kaolinite-dominated to illite-dominated, suite of clay minerals changes from kaolinite-dominated to illite-dominated, typically accompanied by a pronounced decrease in permeability. The Gulf of Mexico sandstones are also initially arkoses. Their composition, however, evolves toward a mixture of quartz and compositionally pure albite. Kaolinite remains the dominant authigenic clay within the sandstones; however detrital clays change from a Na-rich, smectitic mixed layer clay to a K-rich, illitic mixed layer clay. The contrasting diagenetic pathways result from differing mineralogy in the clastic section. The smectite-rich mudstones in the Gulf of Mexico provide a powerful sink for potassium and source of sodium. The resulting low potassium activity results in K-feldspar dissolution; it alsomorexa0» prevents illite formation, while high sodium activity stabilizes albite. The Middle Jurassic clastic section in northwest Europe contains relatively little smectite, thus lacks the potassium sink and sodium source. Sodium activity is low, so plagioclases preferentially dissolve. K-feldspars also dissolve, but the potassium here is available for illite formation.«xa0less