Deposition of deep-water fan systems is, in part, influenced by a variety of shelf margin and slope processes that affect the sediment pathways into the receiving basins. Shelf and slope seafloor topographic changes that resulted from tectonic movement, mobile substrate deformation and/or relative sea level fluctuations, may lead to switching of the sediment pathways leading to deep-water basins and result in depositional pulses of shelf and slope material that interrupt the background pelagic and hemipelagic sedimentation in different areas through time. The sediment pathways of the Permian deep-water deposits in the Tanqua and Laingsburg subbasins seen in outcrops in the southwest corner of the Karoo Basin, South Africa, have been influenced by fluctuations in seafloor topography caused by tectonic forces associated with the adjacent fold thrust belt. The two subbasins have geologically near-contemporaneous formation and filling, and in part contain deep-water sediments. Studies on these deep-water deposits have allowed reconstruction of the shelf and slope environment. Tectonic compression in the area has led to the formation of a basin floor high (anticlinorium) that separates the subbasins and influenced sediment transport. The Tanqua subbasin has developed into a broad, open basin that received five stacked submarine fan systems, while the Laingsburg subbasin has built into a deeper, longer and narrower basin that also has received five stacked submarine fan systems. Petrologic and microprobe analysis of the sandstones in the submarine fan systems indicate that they have come from a common source area that is an extended distance away (as great as 400 km). The subbasins, most likely, share a single shelf edge canyon and slope transportation system that involves enough length to allow for switching of the slope pathways through the evolving topographic highs and lows over time.

Understanding the interactions of the source-to-sink sedimentary system is necessary to place a deposit in a receiving basin in its proper depositional perspective. This is true for any part of the source-to-sink system, whether it deals with fluvial, deltaic, shelf, slope, or deepwater, especially when the sink is now in the subsurface. One should look updip as well as down-dip to understand better the environment of interest. Critical information to classify the variety of submarine fans, such as the range of grain size and the distribution of sand, can be found in the time-equivalent deltaic environment at the shelf margin. Four major factors can be identified that influence all parts of the source-to-sink system: tectonic activity, climate, relative sea level fluctuations, and sediment characteristics. Tectonic activity, both local and regional, comprises the primary factor. Pre-depositional tectonic movements guide the location and elevation of the sediment-providing mountains, climatic conditions for weathering and precipitation, types of subaerial transport, development of the coastal plain and its delta (if any), the shelf width, slope characteristics, and the receiving deep-water basin characteristics. Climate influences several attributes, including sea level fluctuations and sediment transport to the coast and beyond. Sea-level fluctuations, controlled by large-scale tectonic activity and/or climatic changes, influence subaerial and coastal processes. During rising, high relative, or global sea level, coastal plains become submerged and sediments are stored on the continent and the shelf. During the lowering and initial rising stages, sea level can result in shelf-edge deltas or the feeding of sediment directly to a deep-water basin. The maturity of the sediment is strongly influenced by the type and duration of subaerial transport, where the majority of the mechanical and chemical weathering takes place. During subaqueous transport, significant amounts of clay-sized sediment facilitate transport of fine-grained sands far into the ocean basins. Whether deep-water sediments are coarsegrained or fine-grained, the coastline dictates if the feeding sediment source is canyon-fed or delta-fed. Shelf margin deltas are typical for fine-grained sediment and are normally located on wide, low-gradient shelves. Lateral switching of the delta establishes a new location for the entry point of the sediment to a deep-water basin. Shelf margin deltas on narrow shelves do not always exist during low sea-level periods.

Abstract Outcrop sections containing excellent physical and biogenic sedimentary structures within the Late Permian Ecca Group are exposed within the Tanqua Karoo that show the transition from shelf margin delta through to slope and basin floor fans. The Tanqua submarine fan complex comprises six regionally distinct fan systems, five of which form a progradational stack with the sixth fan, to the south, downlapping onto the fifth fan. Progradation of the deltaic deposits across the basin has been in response to a decrease in accommodation space created by relatively high rates of sedimentation within the foreland basin setting. The sedimentology and sequence stratigraphy of the Hangklip Fan represents a shelf margin delta feeding downdip slope fan deposits. Wave ripples, swaley cross-stratification and trace fossils, including Gyrochorte , suggest substantially shallower depositional conditions than slope fan deposits, which are devoid of such features. Erosional slump scars, cutting into laminated shale with chaotic infill of sand intraclasts, point toward slope depositional processes that are not in evidence in the underlying submarine fan deposits.

Abstract The southwestern part of the Karoo basin, South Africa is comprised of two mountain chains (Figures 1,2). The southern branch (Witteberg Swartberg Range) runs east-west, and the western branch (Cedarberg Range) is more or less north-south. Where the two meet is a northeast-trending syntaxis with two anticlinoria. This structural area encloses two foredeep areas, known as the Laingsburg subbasin and the Tanqua Karoo subbasin. The east-west running Laingsburg subbasin is a typical elongated foredeep. Later tectonic activities tilted this basin, with the result that the layers are close to vertical. Between the western branch and the anticlinoria is the location of the Tanqua Karoo subbasin. The outcrops of this basin now cover an area of 650 km2 (250 mi2). The basin is Permian age ( Figure 3 ; Wickens and Bouma, 2000 ). Gradual sinking of the basin has resulted in a 7–8-km (4.36–5.0-mi)-thick package of younger deposits overlying the deep-water sediments. A later rebound eroded overburden, with the result that the Permian deep-water deposits became exposed. The outcropping area is about 34 km (21 mi) long and has no tectonic dip in the north-south direction. The visible width varies from 8 to 12 km (5–7.5 mi), and shows 1–3° tilt to the east. Lack of vegetation exposes large outcrops along the western and southern sides, as well as in the middle along the Gemsbok River ( Figure 5 ). The outcrops make it possible to observe the depositional changes in the downslope direction. The subbasins formed during the Permian compressional collision

Abstract The eastern 2 km (1.2 mi) of Fan 3 outcrops represent channel deposition at the base-of-slope (Figures 1, 2). Figure 1 shows details of the contacts between channels while Figure 2 shows approximately 1200 m (3937 ft) of the outcrop with multiple stacked and laterally offsetting channels that are best described as nested channels. The Ongeluks outcrop area is the transition from the feeder canyon to the deep-water basin. As the canyon flares out and the gradient lessens, density currents move from side to side, filling topography and cutting into pre-existing channels. There are 19 partial channels in the outcrops; most are less than 300 m (967 ft) in width (see Figure 2 ; the summary tables). Many of the sand-filled channels have shales or silty shales lining the channel bases, but in some cases local scour has removed the shale leading to sand-on-sand contacts. The silty shales also occur within the channels and most are laterally discontinuous. The measured sections and paleocurrent directions for this outcrop are from studies by O. van Antwerpen (1992).

Abstract The Fan 5 outcrops at Bloukop Farm include channel and thin-bed deposits. Initially, this outcrop was interpreted as consisting of middle-fan channel sandstones overlying thin-bedded outer-fan sheet sandstones. Closer examination (and clearing of the outcrop face) revealed that the massive channel sandstones were locally coeval with several of the thin beds (see Figures 4, 5) suggesting channel and levee deposition (Bouma and Wickens, 1994). Kirschner (1999) conducted a detailed study of the area (also reported in Kirschner and Bouma, 2000) that supported this interpretation. Two channels were recognized in the outcrop. channel 1 is the lower channel that shows the connectivity of two of the levee thin beds with the channel sandstones. Most of the other beds are truncated by the channel illustrating variable times of channel cutting and thin-bed deposition. Both of the channels are shallow and wide, and they mostly have erosional contacts with the underlying deposits. And although the channels commonly overflowed (sand-rich sediments form poor and low-relief levees), direct contacts between the channel and thin beds are rare due to deposition, scour, and subsequent fill. The average grain size of the channel deposits is slightly larger than the levee deposits. Paleocurrents of ripples indicate a northerly transport direction within the channel while the layered levee deposits have a divergent 35° offset direction on both sides of the channels. Common sedimentary structures include parallel laminations, current ripple laminations, and climbing ripples; climbing ripples are common at this location ( Figure 5 ).

Abstract Kanaalkop, located on the Klein Gemsbok Fontein Farm, contains parts of three of the five fans of the Tanqua Karoo. Fans 1 and 2 are thin and represent parts of distal fans. Fan 3, the subject of this paper, occurs in a middle fan location ( Figure 1 ). The Fan 3 outcrop includes a complete channel cross section and levee-overbank deposits of three precursor channels. The most important part of the outcrop, the fill of the upper channel, is 508 m (1667 ft) wide and 19 m (62 ft) thick in the center. The sandstone layers in the axis of the channel are amalgamated. In the channel margins, the fill thins and the sandstone beds become interbedded by shales. The channel center also has two small erosional scours: a 0.75 m (2.5 ft) cut on the left and 0.50 m (1.6 ft) cut on the right. The center of the channel is mostly parallel to the underlying bedding. Thin beds underlie the entire channel complex ( Figure 2 ). Work by Kirkova (1998) indicates that these thin beds represent overbank deposits from three other channels (not present in this outcrop). The three intervals are labeled a, b, and c in Figure 2 . Note that there are more thin beds underlying the margins of the main channel where the channel thins laterally. Overall, there are general fining- and thinning-upward trends in these units and there is also an offset (possibly due to compensating bedding) in the thickening and thinning of the thin-bedded units.

Abstract Excellent exposures of Fans 3 and 4 in an L-shaped outcrop are present at the Klip Fontein Farm. Some parts of Fan 5 can also be seen. The east-west arm is 1800 m (5900 ft) long and the north-south arm is 1000 m (3280 ft) long. Fan 3 is fed from the south, making the east-west arm a strike section. Fan 4 is fed from the west and shows a dip section on the east-west outcrop. Fan 5 is fed from the south. The outcrops are a classic example of a high net-to-gross, fine-grained, amalgamated sheet package deposited in the outer fan. The strike and dip sections show very similar features. Fan 3 is about 13.5 m (44 ft) thick, and Fan 4 is 54–60 m (177–197 ft) thick. The shale separating the two fans has a thickness of about 16 m (54 ft). Both fans represent outer-fan sheet sands (depositional lobes). This paper will concentrate only on Fan 4. At first view, the sandstone layers appear to be parallel. However, detailed drafting of shale contacts shows rather flat bottoms and the convex upper contacts show a gradual lateral thinning of the sandstones. Fan 4 shows laterally continuous sandstone and shale beds and occasional amalgamation in the center of succeeding sandstone beds. The thin interbedded shales are comprised of alternations of very thin sandstones and shales. Plant fragments are common on the top of most silt-rich shale laminae. Thickening-upward sequences and some incomplete Bouma sequences can be detected. Paleocurrent directions vary widely due to bathymetric compensation.

Abstract Fan 3 on Kleine Riet Fontein Farm shows two interesting outcrops separated by shale scree. At the southern outcrop, channel fills can be observed, and in the northern outcrop are levee-overbank deposits with a number of thicker, massive sandstones with net-to-gross (N:G) near 1.0 that can be recognized by their light color. They are interpreted as crevasse splays. The north-south-oriented outcrop probably makes only a small angle with the overall north-south paleotransport direction of Fan 3, making it difficult to obtain a good cross view of the channel margins. Nevertheless, the outcrop shows important parts of channels and levees belonging to a fine-grained turbidite system. The location of both outcrops is shown in chapter 75, this volume. Figures 1A-C reveal three channels separated by thin shales and very thin sandstones. The shaly separations thicken to both sides, indicating that the erosion of the underlying channel was often strongest in the central zone of the channels (see upper channel, Figure 1A ). This photograph shows thin-bedded, sand-rich layers separated by silt-rich shales. The picture in Figure 2 does not reveal the thickness of the sand layers. Figure 3B shows four light-colored, massive sandstone layers, which are interpreted as crevasse splays. The massive central part of each channel shows some bedding when observed close-up. Layers are separated by thin shale and/or thin sandstone layers. Both sides of the massive part become thinner layered and thin-bedded. A succession of sandstones and silt-rich shales becomes more distinct toward the ends.

Abstract Although the cleanly visible portion of the outcrop at Kleine Gemsbok Fontein Farm is quite small (36 m [118 ft] long) and lacks third dimensionality, it is a good example of the transition from thicker amalgamated sheet sandstones to thinner sandstones in a very short distance. In addition, details of amalgamation are well preserved in this high net-to-gross, fine-grained outer-fan setting. The Fan 2 outcrop, interpreted as outer-fan sheet sandstones, is an example of local connectivity in an overall nonamalgamated outer-fan layer sheet sand deposit (Rozman, 1998, 2000; Bouma and Rozman, 2000). This small outcrop, orientated northeast-southwest, shows massive, amalgamated sandstones that change laterally to bedded silty shales and thin sandstones. Some of the layers show vertical connections where the shale is locally eroded and filled with sandstone ( Figure 1 , arrows point to the locations). This outcrop is oriented at 45° with respect to the paleocurrent direction. No third dimension is available to determine whether these sand-on-sand contacts are localized or extensive, or in which direction they are oriented. If they are extensive, they may represent filled gullies (furrows).

Abstract Siltstones, mudstones and shales have been studied mainly with regard to general transport-deposition processes and clay mineralogy. A small group of investigators, with differing backgrounds, have worked on these fine-grained deposits. While there have been a number of resulting publications, they are spread out over a wide array of geological literature. Recent studies on deepwater deposits from cores and outcrops indicate that the presence of finer-grained deposits greatly affect the fluid flow properties of deepwater reservoirs. Further analysis indicates that the majority of these finer-grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales. Studies on these deposits have indicated that they are often comprised of graded fine silt laminae sandwiched between films of clay minerals, quartz dust and organics. Characteristics and rock properties of these deposits, which resulted from a variety of depositional processes, are just beginning to be understood. Depending on the transport-deposition processes, the architecture of the deposit will have different 3D extents and continuity as well as varying rock properties. Stratigraphic prediction of the position and dimensions of the fine-grained beds can indicate whether the deposits will be a barrier or a baffle to fluid flow or a possible reservoir for natural gas. Minor variations in depositional style and other characteristics can result in more differences than presently assumed.

Abstract The continental margin of the northern Gulf of Mexico is characterized by a very complex morphology due to the interactions between sedimentary and halokinetic processes. Sediments deposited on the margin during the last glaciation provide an excellent opportunity for the study of depositional processes of fine-grained sediments in a structural complex deep-water environment. This study is based on detailed analysis of long sediment cores and high-resolution geophysical data from two areas of the northern Gulf of Mexico: Bryant and Eastern Canyon Systems and the Atlantis Discovery. At least three sedimentary provinces existed on the continental margin during the last glaciation. The Mississippi Canyon and Fan resulted from the building of the Mississippi River delta at the shelf-margin during the last glaciation. Turbidity currents flowing through the Mississippi Canyon at this time contributed to the continued development of the preexisting Mississippi Fan. The Texas and Louisiana continental slope province characterized by numerous intraslope basins. Deposits from the last glaciation consist of hemipelagic sediments interbedded with finegrained (silty-clay to clayey-silt) turbidites that thin and fine downdip. These deposits were produced by low-density turbidity currents that resulted from the depositional segregation (deposition of the coarsest material at the most proximal locations) of large turbidity currents initiated on the outer shelf and/or upper continental slope. Thick (up to 40 m), structureless mud deposits (unifites) on the floors of intraslope basins most likely resulted from the partial deposition of long-lasting (1.5-3 months), low-density turbidity currents. The continental rise and lower continental slope province of the northwest Gulf of Mexico has fine-grained, layered sediments that are siltier than those of the fine-grained turbidites from the continental slope province, and occasionally reveal a lenticular to wavy nature. The layered sediments are interpreted as deposits of turbulent sediment flows whose site of deposition is controlled by westward flowing bottom currents. The currents pirate the finer-grained sediment load (upper part and tails) of turbidity currents flowing through the Mississippi Canyon to the Mississippi Fan and relocate them within this province. Turbidity currents were most abundant between 32-28.5 ky BP and 28.5-21.5 ky BP. The first time interval coincides with the development of a major deglaciation event that led to highly increased river discharges. The second time interval coincides with the drop of the sea level to the shelf edge, and the production of shelf margin deltas during the last glacial maximum.

Abstract The goal of this research is to develop a predictive model for use in hydrocarbon exploration and risk analysis/mitigation that permits estimation of the sealing capacity of marine mudrocks. Research to date has concentrated on Cretaceous shales of the Western Interior foreland basin in Colorado and Wyoming and Eocene shales in the Ventura basin, California. This research focuses on Eocene rocks of the central Pyrenean foreland basin, Spain. Outcrop mudrock samples were collected from areas around Ainsa, Broto, Undues, and Anso in northeastern Spain. Geologically the areas sampled include the Ainsa basin slope, Broto base of slope, and the Jaca outer fan and basin plain in the south-central Pyrenees. Extensive research to define stratigraphic relationships, depositional environments, basin type, and sandstone body geometry have been undertaken, however the nature and sealing capacity of included mudrocks have been largely ignored. The Eocene Ainsa basin, from which most of our samples were taken, originated as a foredeep ahead of a thrust ramp and evolved into a piggy back setting as the thrust migrated basinward. This foredeep was filled with up to 4000m of slope sandstones and mudrocks, had a source area to the southeast and a basin plain (the Jaca basin) to the northeast. Sealing capacity of these mudrocks was determined by mercury injection-capillary pressure (MICP) measurements conducted by Poro-Technology on 43 samples representing a range of slope, base of slope, and basin floor environments. Capillary pressure curves generated during mercury injection have been used to evaluate sealing capacity by equating it to the pressure required to achieve 10%mercury saturation. Pore throat diameter was determined from data generated during the MICP analyses. Bioturbation was characterized on a qualitative scale of 0 to 6. Mean quartz grain size was determined by measuring the apparent long axes of thirty quartz grains per sample. Total organic carbon (TOC) and CaCO 3 were determined by analytical techniques in the soils laboratory at Colorado State University Samples analyzed range from laminated and bioturbated, calcareous mudstones to silty biomicrites and calcareous siltstones. Some samples contain thin, normally graded, laminations of silt-size quartz grains. MICP values at 10%saturation range from 500 PSIA to more than 60,000 PSIA. Despite the high degree of variability in samples from a single outcrop and from each sampled area there is a general correspondence with geographic and sequence stratigraphic setting. Furthermore, significant correlations exist between MICP and sample porosity, pore aperture diameter, standard deviation of pore diameter, and TOC. As expected sample porosity, pore diameter and standard deviation of pore diameter are inversely related to sealing capacity. TOC is directly related to sealing capacity. Degree of bioturbation is more variable in samples with low sealing capacity and is generally lower in samples with high sealing capacity. There are no apparent correlations between MICP and permeability, grain density, average quartz grain size, standard deviation of quartz grain size, quartz grain roundness, standard deviation of quartz grain roundness, or CaCO 3 content. From a depositional setting and sequence stratigraphic viewpoint proximal slope deposits have the lowest average sealing capacity, highest porosity and permeability, highest pore aperture diameter and most poorly sorted pore diameters, highest overall grain size and degree of bioturbation, and lowest TOC. Basin floor deposits have the highest average sealing capacity, lowest porosity and permeability, smallest pore aperture diameter, and best sorted pore diameters, lowest overall grain size and degree of bioturbation, and highest TOC. Compositional variables, other than TOC, are less important than textural variables in determining the sealing capacity of these mudrocks. Factors that influence the relative sealing capacity of these carbonate-rich mudstones are generally similar to those for non-calcareous shales from other foreland basins and include overall grain size, degree of disruption of depositional fabric by bioturbation, pore throat size and sorting, TOC, and depositional setting within the basin. The variables that most strongly favor high sealing capacity are most likely associated with deposits of deep water anoxic environments, hence the common association between good seals and upper transgressive systems tract deposits and condensed sections.

Abstract The Newark East Barnett Shale Field located within the Fort Worth Basin of North Texas has now developed into the largest gas field within the state of Texas. The productive formation is the Mississippian Barnett, an organic rich, very dense, black shale with 3-5% porosity, less than .001 md permeability and greater than 140 BCF per square mile of gas in place. Newark East Field has produced in excess of 800 BCF of natural gas, over 220 BCF in 2002 alone, and is currently producing greater than 800 MMCF per day from over 2,350 wells. There are over 60 companies participating in the play with 55 rigs actively targeting the Barnett. The Newark East Field core area is now approaching full development on 40 acre spacing. The current field limits are being tested by wells targeting the Barnett Shale to the east and northeast into the deepest portion of the basin adjacent to the Muenster Arch, to the north toward the oil window and to the south into the Fort Worth metropolitan area. The greatest challenge facing the Barnett play expansion lies to the west and southwest into western Wise, Parker and Johnson Counties where the underlying Ordovician tight limestone frac barriers, which are viewed as key to successful wells, are absent. Several wildcat wells have tested the Barnett to the south and west utilizing both vertical and horizontal technology with varied results. Clearly the conventional technology developed within the core area will not be applicable to all of the expansion and exploration areas. A greater understanding of the Barnett Shale as a reservoir, as well as increased study of the frac barriers below, above and within the Barnett Shale are now necessary. Armed with this knowledge, the drilling and completion technology can be developed to allow for the successful expansion of the play.

Abstract Sealing characteristics of marine shales are among the least understood aspects of petroleum systems. Petrophysical measurements indicate that the largest interconnected pore throats ultimately control seal behavior. Pore throat diameter, determined from mercury-injection capillary pressure (MICP) analysis, is influenced by numerous factors, including: composition (total clay content, and organic enrichment), fabric and texture (fissility, silt content and bioturbation), and diagenesis. Data from deepwater wells in the Gulf of Mexico and offshore Angola document the variability of shale microfacies and sealing character in marine depositional settings. This variability can be quantified and predicted where considered within the context of sequence stratigraphy and shale sedimentology. The analyzed Tertiary-aged marine shales record deposition in middle to lower slope paleoenvironments and are interstratified with sandstones representing lowstand fan lithofacies. Six shale microfacies can be defined based on differences in fabric and petrophysical properties: 1) well-laminated, slightly silty, organically-enriched shales; 2) moderately silty, partially laminated shales; 3) moderately silty mottled shales; 4) very silty mottled shales; 5) very silty shales interlaminated with siltstones and very fine sandstones; and 6) calcareous shales and claystones. Shale types 1, 2 and 6 consistently exhibit better than average seal capacities. Shale types 3 and 4 are moderate to good seals, and Type 5 shales are typically poor to very poor seals. Top seal capacity increases as clay content increases and decreases as the content of detrital silt increases. Depositional fabric appears to exert primary control on seal character, and early marine cementation can significantly enhance seal capacity. The texture and composition of marine shales vary systematically within depositional sequences and correlate with variations in sealing capacity. Silt-rich shales in highstand and lowstand systems tracts have 10% non-wetting (MICP) saturations that are consistently low relative to those of transgressive shales. The highest 10% non-wetting mercury-injection capillary pressure (MICP) saturations values correspond to transgressive and condensed shales containing significant percentages of authigenic carbonates. Shales occurring within the upper part of third-order transgressive systems tracts are typically excellent to exceptional top seals. These finely laminated, silt-poor, transgressive shales commonly have elevated percentages of organic matter and authigenic iron minerals. Seal capacity generally increases, basinward, from near shore to distal offshore marine settings. Where a transgressive shale is the controlling top seal for a lowstand reservoir, a thick waste zone commonly separates the seal and the subjacent reservoir.

Abstract Fluid muds are high-concentration near-seabed suspensions of fine sediments, and have been recognized in coastal sedimentary systems for many years. However, in the past decade, fluid-mud development and transport have been identified in an increasing number of settings as important mechanisms for marine dispersal of fluvial sediments. It has been shown that the combination of high-energy benthic hydrodynamics and sufficient fine sediment can result in cross-shelf gravity-driven flows (on very low slopes) that can blanket hundreds of square kilometers to thicknesses exceeding 10 cm. The sedimentary fabric that results from gravity-driven flows consists of a stacked pattern of predominantly fine-grained, fining-upward packages. The resulting morphology of the shelf can be a clinoform, with maximum deposition occurring on the foreset (convex-upward) region. The western Louisiana inner shelf has been experiencing fluid mud deposition in response to increased fine sediment supplied by the Atchafalaya River since ∼1950’s. A review of observations collected during recent studies of the Eel and Fly rivers, and their similarities to the Atchafalaya shelf indicate that wave-enhanced gravity driven flows are responsible for the sedimentary features and clinoform morphology present along the Chenier plain coast of Louisiana.

Abstract Geoscientists and engineers have long appreciated that intrareservoir mudstones may affect oil and gas recovery significantly. Mudstones have particularly significant effects in reservoirs with low net-to-gross ( e.g., overbank turbidites) or if reservoirs have highly continuous mudstone layers ( e.g.., tide-influenced deltas). Interdisciplinary outcrop and shallow analog studies formulated and tested models for the spatial distribution of mudstones in deltaic sandstones. At scales from decimeters to hundreds of meters, mudstone length distributions may be described using gamma distributions. This approach is data rich, computationally tractable, and provides direct links to covariance geostatistics commonly used in reservoir modeling. These outcrop observations have been related to and validated using shallow, high-resolution ground-penetrating radar surveys. Experimental design and flow simulations assessed the importance of the intrareservoir mudstones: in a tide-influenced delta the mudstones controlled flow, whereas the mudstones were nearly inconsequential in a marine-influenced distributary point bar.

Abstract Overcoming wellbore drilling instability in shale formations will always present one of the major challenges to drilling engineers. It is well established that the pore pressure differential between the drilling mud and the drilled rock formation is governed by the diffusion process which is a time-dependent phenomenon. In addition, shales are highly chemically active and they swell when brought in contact with aqueous drilling fluids. Chemical effects arise from imbalance in chemical potentials of the species in formation pore fluid and wellbore drilling fluid. Thus, the downhole chemical reactivity (mud/shale) is also a time-dependent variable. This work presents the development of an analytical formulation to analyze the wellbore stability in chemically active sound and fractured shale under the framework of Biot’s theory of poroelasticity. The majority of the existing wellbore stability prediction models are based on the solutions obtained using single-porosity poromechanics theory which considers the rock formation to be homogenous non-fractured and with one degree of porosity distribution. However, they are not applicable when the rock formation is fractured. The dominant characteristics of fractured rock formations, i.e., the coupling between various induced hydro-physico-mechanical processes and inter-porosity flow can be well represented under the realm of dual-porosity poromechanics. In this work, the approach to modeling the porochemoelastic behavior of wellbore drilled in fractured shale is formulated as an extension of Biot’s poromechanics theory that fully couples the perturbation in stress, pore pressure and solute (salt) concentration in a consistent form. The shale is modeled as an imperfect semi-permeable membrane which can allows partial transport of the solutes. The pore fluid is assumed to be a binary solution that contains only a solvent (water) and a solute (salt). The transport equations for both solute and solvent take into account the hydraulic convection, chemical osmotic, advection and solute diffusion (Fick’s law). The magnitude of the chemical interaction is controlled by the membrane efficiency. The model is applied to derive a solution for an inclined/horizontal wellbore in isotropic sound and fractured shale formation subjected to the three dimensional in-situ state of stress. Mud weight window analyses are also illustrated in this work.

Abstract Stratigraphic correlation of deep water siliciclastic sequences generally is based on biostratigraphic, palynological, and lithologic markers. However, in sequences that are barren of, or have limited fossil assemblages along with inconclusive lithologic correlation, the chemical composition of the silts and shales in the stratigraphic section can be used to determine potential tie points in the basin fill. This chemical composition can be used to correlate sedimentary packages across a basin or even potentially between adjacent basins. Successfully used, correlation of the chemical composition of shales and silts can give a fuller understanding of the timing of the basin fill and depositional environments.