Studies of modern desert dune fields allow geologists to draw conclusions about the controls that govern the development of spatial patterns of arrangement of desert landforms. This knowledge can be applied to predict the likely arrangement of architectural elements in preserved ancient desert successions. This serves as the basis for the development of more sophisticated facies, architectural-element and sequence stratigraphic models that can be applied in reservoir geology.
This study presents a series of ten bespoke facies models that demonstrate different types of aeolian-fluvial interaction documented from dune-field margin settings. These ten semi-quantitative models have been developed based on analysis of modern and ancient systems, and via comparison of literature-derived case-study examples of ancient successions using a meta-analysis approach. The presented facies models account for the nature and origin of stratigraphic complexity present in aeolian dune-field margin successions that arose in response to the combined interplay of a series of autogenic and allogenic controls.
From an applied perspective, mixed aeolian and fluvial successions are known to form several major reservoirs for hydrocarbons, including the Permian Unayzah Formation of Saudi Arabia. However, quantitative stratigraphic prediction of the three-dimensional form of heterogeneities arising from aeolian and fluvial interaction is notoriously difficult: (i) interactions observed in one-dimensional core and well-log data typically do not yield information regarding the likely lateral extent of sand-bodies; (ii) stratigraphic heterogeneities of these types typically occur on a scale below seismic resolution and cannot be imaged using such techniques.
Understanding the nature and surface expression of various types of aeolian and fluvial interaction, and considering their resultant sedimentological expression, is important for prediction and interpretation of preserved deposits of such interactions that might be recognized in the ancient stratigraphic record. Assessment can be made of the spatial scale over which such interactions are likely to occur and this has applied significance; the developed facies models facilitate the prediction of net reservoir sandbody dimensions from subsurface successions by constraining the geometry and lateral and vertical connectivity of sand bodies for specific desert system types. Assuming layer-cake correlations between neighbouring wells within stratigraphically complex reservoirs composed of mixed aeolian and fluvial facies is inappropriate; instead, a range of bespoke facies models should be utilized, each of which considers possible stratigraphic configurations and each of which has implications for likely reservoir performance.
A morphological model to predict the formation of tidal dunes (sand waves) and their migration is described. The analysis assumes that the appearance of the bedforms is due to the growth of the most unstable component of an initially random perturbation of small amplitude, forced by a uniform tidal flow. Results are presented for a unidirectional tide, even though the analysis can be easily extended to an elliptical tide. The wavelength of the fastest growing component of the bottom perturbation is found to fall in the range of the wavelengths of the tidal dunes observed in the field. The forcing tidal flow contains both the M4 and Z0 constituents, which are superimposed to the main M2 constituent and the bottom forms are found to migrate either in the direction of the residual current or in the opposite direction, depending on the parameters of the hydrodynamic and morphodynamic problems. A comparison of the theoretical results with field data shows that the model provides fair predictions of both the geometrical characteristics of the tidal dunes (sand waves) and of their migration speed.
When strong tidal currents are present, tidal bedforms i.e. undulations of the sea bottom are observed which are characterized by wavelengths much longer than the water depth. Two different bedforms are observed, namely tidal dunes (also named sand waves) and sand banks (also named tidal ridges). The wavelength of the tidal dunes is of the order of ten times the water depth and the crests of the bottom forms are almost perpendicular to the oscillating tidal current. Moreover, tidal dunes are often observed to migrate in the direction of the residual current, even though example of bottom forms migrating against it do exist (Allen, 1994; Stride, 1982). On the other hand, the wavelength of sand banks is of the order of hundred times the water depth and their crests are almost aligned with the main direction of the tidal current, usually forming small counter-clockwise angles. Moreover, sand banks hardly move (Allen 1994; Dyer & Huntley, 1999). The height of tidal dunes is of a few metres, even though there are tidal dunes which are characterized by height up to tens of metres (Van Landeghem et al. 1999). On the other hand, sand banks have much larger heights which may be a significant fraction of the local water depth. Hulscher et al. (1993) pointed out that the presence of both tidal dunes and sand banks is highly correlated with the presence of intense tidal currents. While the migration of sand banks is negligible, tidal dunes have significant migration speeds which can reach tens of metres per year. The predictions of the migration of tidal dunes is relevant for the offshore industry and the maintenance of navigation channels. Indeed, a pipeline, on a sea bottom characterized by the presence of tidal dunes, may experience large free spans which can induce oscillations, fatigue phenomena and, under certain conditions, even the buckling of the pipeline or its break. Therefore pipelines, in areas where tidal dunes are expected to be present, are often buried at a significant depth below the level of the expected troughs of the dunes, which is an expensive operation. Moreover, migrating tidal dunes can move large volumes of sand in shipping channels thus decreasing the local water depth and asking for periodic dredging activities. Finally, migrating tidal dunes can decrease the local sea bottom level and mine the stability of offshore structures, as wind mill parks and oil platforms. The horizontal length scales of sand banks and sand waves are quite different from the length of the tidal wave and Hulscher et al. (1993) suggested that the appearance of tidal dunes and sand banks might be due to a free instability of the morphodynamic system describing the interaction between the currents induced by tide propagation and the cohesionless sea bottom. Hence, they concluded that the process which leads to the formation of these bottom forms can be investigated by means of a linear stability analysis. Stability analyses aimed at investigating the formation of tidal dunes and sand banks were formulated by different authors (Huthnance, 1982a,b; Hulscher et al., 1993; Hulscher, 1996; Besio et al. 2006). However, these models consider symmetrical tidal currents and the bottom forms predicted by the stability analyses do not migrate. The model by Besio et. al (2006) is presently extended to take into account more than one tidal constituent, thus allowing to predict the speed of migration of tidal dunes.
The first study (June, 2010 - January, 2011) used GPR and vibracores to investigate variations within the internal structure of various-sized foredunes along a 2.5 km length of the non-driving (restricted vehicular access) section of PAIS. Three representative foredunes with different heights were selected within the storm impact regime proposed by Sallenger (2000). A small (1.79 m), intermediate (2.69 m) and large dune (3.77 m) represent inundation, overwash and collision regimes, respectively. The second preliminary study (May, 2013) used the EM induction method to measure spatial variations in apparent conductivity across the intermediate dune site surveyed by GPR in 2010-2011. A 100 m long EM transect was conducted in close proximity to the 2010-2011 GPR survey area. Additionally, a new GPR survey was conducted along the same EM profile for comparison with the EM data as well as the GPR profile taken in 2010-2011. Currently, we plan to conduct several extensive GPR and EM surveys at regularly-spaced intervals ( 15 km apart) within the driving (vehicular accessible) section of PAIS and across the Laguna Madre wind-tidal flat system.
Influenced by the success of shale gas production worldwide and to meet requirements for clean energy supply, a multidisciplinary team of petroleum specialists was established in Saudi Aramco. Meeting the growing requirement in industrial consumption and especially electricity production is driving force for developing unconventional gas reserves. "The initial focus is in the northwest and in the area of Ghawar, where gas infrastructure exists. Initial knowledge building from similar plays in North America is being supplemented with internal technical studies and research programs to help solve geological and engineering challenges unique to Saudi Arabia and to locate specific wells planned for 2011. The company is innovatively combining knowledge and research to maximize gas reserves and production from conventional and unconventional resources in order to meet growing domestic demand.?? 
During years 2010 - 2011 major international petroleum industry players - Schlumberger, Halliburton and Baker Hughes - were invited to share their experience in a series of workshops held in Dhahran. Exchange of expert ideas developed into appreciation of complexity of the shale gas reservoir and helped to identify the scope of work for the first Silurian Qusaiba shale gas well. The SHALE-1 well was drilled in 2007 as a gas exploration well. Recent drilling and geophysical data obtained in the well were beneficial for detailed sidetrack and fracture stimulation design.
The Multidisciplinary Saudi Aramco - Halliburton SHALE-1 task group was established and positioned in Dhahran. This allowed them to have regular face-to-face meetings and improve the most critical criteria of any new venture - communication. The draft work plan was developed 8 months before actual operations commenced on the well site. Thorough examination of the draft work plan progressed to the final work plan with a number of improvements. For example, "R?? Nipples were dropped from the monobore 4-1/2?? completion string. The Frac Stimulation design was fine-tuned, involving expertise from Saudi Aramco and Halliburton. The Complete Well on Paper exercise involved over 25 specialists from both sides and helped to rectify remaining completion/stimulation design issues, and put everyone on the same page in terms of the work program. Well site operations commenced in May 2011; the well was successfully re-entered and window cut in 7?? liner. An S-shaped 5-7/8?? hole was drilled in the direction of minimum horizontal stresses, to the required depth in Qusaiba Shale with a maximum DLS of 4°. The well was completed with 4-1/2?? cemented liner and monobore 4-1/2?? string to surface. The Hot Qusaiba interval was perforated; frac stimulated with mixed results and successfully flowed. A temporary isolation FasDrill plug was set above the perforation interval. The Warm Qusaiba interval was perforated; successfully frac stimulated and flowed with mixed results. Finally, the FasDrill plug was drilled out with CTU and both intervals flowed and required production log runs.
All targets set for the SHALE-1 re-entry well were successfully achieved and the well was suspended for future utilization as an observation well.
Arctic regions have been determined to be particularly sensitive to a warming global climate both on the basis of climatic modeling and observation of dramatic changes in arctic landscapes and sea ice. As early as the 1990s, air temperatures in interior northwest Alaska were warming at a rate of 0.75 °C per decade. The resulting thawing of permafrost causes significant damage to buildings, roadways, and can lead to increased mass wasting (e.g., active layer detachments and thaw slumps) by melting the soil ice that "cements?? the grains together to resist soil movement, as well as ground subsidence. These climate-induced ground movements can threaten infrastructure, such as road, bridges, and pipelines, either by direct physical damage or indirectly, such as through changes in drainage patterns, increased risk of flooding and forest fires.
Because these geohazards often occur in remote locations with harsh weather conditions and limited access, and the precursor conditions for initiating them can occur gradually, methods for remotely monitoring changes in ground conditions and estimating ground failure risks have significant engineering and economic value. The research described in this paper addresses this need by developing techniques for detecting changes in permafrost and seasonally frozen soil terrains using satellite and airborne remote sensing data, and combining these data with mathematical models to estimate the risk of ground failures due to soil thawing. The methodology consists of combining multiple sources of satellite-acquired synthetic aperture radar (SAR) data with high resolution optical-band data and aerial photography to map frozen ground and associated changes in soil moisture, and to detect vertical and lateral ground movements.
The remote sensing data interpretations along with traditional soil and vegetation mapping are used to inform mathematical models of permafrost and frozen soil stability. These models are used to develop maps of the probability of ground movements associated with permafrost degradation and seasonally frozen soil under current and future climate conditions. The models and slope stability risk algorithm were applied to a portion of Kobuk Valley National Park, Alaska for which soil and vegetation land cover maps were available. The cryosphere model results suggest that the same relative change in active-layer thickness occurs across the landscape, but warmer locations experience a larger absolute change in active-layer thickness and may experience permafrost loss as a consequence. The slope failure risk algorithm indicates that the upland areas are most susceptible to slope failure, particularly south-facing slopes, but low-cohesion low-land soils and steep river banks are also susceptible to failure.
The complex interplay between depositional facies and diagenesis in dolostones makes calculating petrophysical properties from wireline logs challenging. Complex pore geometries and mineralogies control rock petrophysical properties. The complex mineralogy of some dolostone reservoirs, moreover, has profound effects on wireline-log measurements. Therefore, equations for calculating porosities and saturations must be tailored to specific pore-geometry/mineralogy combinations. If dolostone reservoirs are divided into petrophysical/mineralogical facies of similar depositional and diagenetic textures - and, thus, similar pore geometries and mineralogy - empirical equations that apply specifically to that geologically identified facies can be developed. These equations yield more accurate calculations of porosity and water saturation.
Our examples from Permian shallow-water dolostone reservoirs of the Permian Basin, southwestern United States, demonstrate analytical approaches for calculating petrophysical properties in these complex rocks. Four general petrophysical/mineralogical facies characterize Permian shallow-water dolostone reservoirs: (1) subtidal, mud-dominated dolostone; (2) subtidal, grain-domi-nated dolostone; (3) dolomitic and siliciclastic peritidal rocks; and (4) diagenetically altered, subtidal dolostone.
The complex interplay between depositional facies and diagenesis in dolostones presents numerous challenges for calculating petrophysical properties from wireline logs. Complex pore geometries and mineralogies control rock petrophysical properties, and equations for calculating porosities and saturations must therefore be tailored to specific pore geometry-mineralogy combinations. The complex mineralogy of some dolostone reservoirs, moreover, has profound effects on wireline log measurements. If dolostone reservoirs are divided into petrophysical-mineralogical facies of similar depositional and diagenetic textures and, thus, similar pore geometries and mineralogy, empirical equations that apply specifically to that geologically identified petrophysical-mineralogical facies can be developed so that porosity and water saturation can be calculated accurately.
We present some examples from Permian shallow-water dolostone reservoirs of the Permian Basin, southwestern United States, that demonstrate analytical approaches for calculating petrophysical properties in these complex rock types. The four general petrophysical-mineralogical facies that characterize Permian shallow-water dolostone reservoirs are (1) subtidal, muddominated dolostone; (2) subtidal, grain-dominated dolostone; (3) dolomitic and siliciclastic peritidal rocks; and (4) diagenetically altered, subtidal dolostone.
The multiple pore types and associated pore-throat geometries and variations in siliciclastic and calcium-sulfate content, which are characteristic of complex dolostone reservoirs and a consequence of both depositional and diagenetic processes, require that petrophysical calculations from wireline logs be tailored to specific rock types. Simply put, "standard" rock equations generally yield unreliable calculations of porosity and saturation.
It is well established that pore types and pore geometries in carbonate reservoir rocks control petrophysical properties such as porosity, permeability, and saturation1-5. It is also well established that the complex mineralogies in some dolostone reservoirs can have a tremendous influence on wireline tool response6,7. Therefore, petrophysical characterization, and, more specifically, accurate wireline log analysis of dolostone reservoirs, require an understanding of both complex pore geometry and mineralogy. We describe methods of determining petrophysical properties in complex dolostone reservoirs by considering the key rock properties of mineralogy, depositional and diagenetic textures, and pore and pore-throat geometry. We incorporate all of these rock properties into calculations by grouping parts of the reservoir into petrophysical-mineralogical facies. In this paper we draw on examples from Permian (Guadalupian and Leonardian) reservoirs of the Permian Basin in the southwestern United States, although the principles and procedures presented herein have application to dolostone reservoirs throughout the world.
Influence of Pore Geometry on Petrophysical Properties
Petrophysical properties in complex carbonate rocks are mutually interdependent. Porosity-permeability cross-plots and the Archie equation8, which expresses formation resistivity factor (FRF) as a function of porosity, are two of the most commonly recognized examples of this interdependency. It follows that if permeability and FRF vary with porosity, then FRF must be recognized as varying with both porosity and permeability. Similarly, capillarypressure characteristics are dependent on both porosity and permeability as seen in the Leverett j-function. The mutualinterdependence of petrophysical properties is an important consideration for making accurate models of porosities and saturations in complex dolostone reservoirs.
San Andres carbonate reservoirs have long been known to have a high degree of reservoir heterogeneity and poor recovery efficiencies. Fractures are one of several causes of this heterogeneity. The heterogeneity causes unpredictability in water and CO2 flooding. However, the correct placement of horizontal wells can take advantage of this problem.
An integrated reservoir characterization study of the Mabee field incorporating oriented core, Formation Microscanner (FMS) wireline logs, seismic time slices, production character, curvature analysis, and interference testing was used to predict fracture orientation and areas of highest fracture density. These fracture characteristics were then applied to determine horizontal well loca-tion and orientation. Fracture orientation was evaluated through the analysis of oriented core, FMS logs, and interference testing, indicating a fracture orientation of N70W. Analysis of the induced fractures in the oriented core indicates that the direction of maxi-mum horizontal compressive stress is N45E. High fracture density was delineated by curvature analysis, relative seismic amplitude, and areas of higher production. Areas with high curvature corre-spond to areas of high relative seismic amplitude and higher production. The data integration indicates that four areas have high fracture density. The synthesis of fracture orientation and density, along with the production character, indicates the optimal location and orientation of horizontal wells.
Low-permeability San Andres reservoirs of the Central Basin Platform contain significant volumes of remaining oil. The Mabee San Andres field lies on the northeastern edge of the Central Basin Platform (Fig. 1) and is part of the San Andres/Grayburg Platform Carbonate play.1 Ref. 1 reported recovery efficiencies for secondary recovery of approximately 30% and an unrecovered resource of 2.6 billion stock-tank barrels of oil. The low recovery efficiency and still-remaining resource are due largely to the signif-icant amount of heterogeneity found in these reservoirs.
San Andres Platform Carbonate reservoirs are highly hetero-geneous because of the depositional facies, diagenesis, and frac-turing. Ref. 2 described how grainstone bar depositional facies significantly affected the production character in Dune (Grayburg) reservoirs. Ref. 3 described how areas of postdepositional dia-genesis were the most highly productive in the Jordan (San Andres) reservoir. Additionally, fractures have been cited as contributing significant heterogeneity to San Andres/Grayburg reservoirs. Ref. 4 sited fractures in the Arrowhead (Grayburg) reservoir as the reason that tracers broke through in 2 days between a five-spot well pat-tern. Ref. 5 described the influence of fractures in the Keystone East (San Andres) reservoir. Ref. 6 described how fractures in the Chaveroo and Cato (San Andres) reservoirs influenced flow and storage volume. Ref. 7 depicted natural fractures as dominating the permeability character in zones of the Levelland (San Andres) reservoir.
This heterogeneity causes preferential fluid flow and often-early breakthrough in waterfloods. It is also the likely cause of water loss previously unaccounted for in San Andres waterflood operations. Ref. 5 described a northeast preferential flow direction coincident with their interpreted direction of maximum horizontal compressive stress. Ref. 8 cited the Fullerton Clear Fork, Keystone Colby, and Means (San Andres/Grayburg) reservoirs as having east-west preferential flow directions. It is reasonable that this similar preferential flow direction in several fields and several formations is due to open fractures.
Both the direction of open fractures and the location of densely spaced fractures influence how fractures affect production. In this study we combine geologic and engineering information including interference tests, oriented core, Formation Microscanner (FMS) logs, production data and curvature analysis to evaluate the direc-tion of open fractures and the areas where they may be more densely spaced.