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Composite halokinetic sequences (CHS) are unconformity-bounded successions of upturned and thinned strata that form due to drape folding of diapir roofs during passive salt rise. Tabular and tapered CHS have narrow (50–200 m) and broad (300–1000 m) zones of folding, respectively. CHS are originally defined as exposed diapirs bounded by shallow-water strata in La Popa Basin, Mexico. This paper tests the concepts of CHS development at the subsurface, deepwater Auger diapir in the northern Gulf of Mexico. We used 3D wide-azimuth seismic data, well and biostratigraphic data, and structural restorations to interpret and analyzed 11 well-imaged Pleistocene CHS that correlate around the diapir. The lower and uppermost flanks are characterized exclusively by tapered CHS, with wide zones of thinning (240–660 m) and broad taper angles (41°–75°). In between are four discrete CHS with mixed tapered and tabular geometries, with the latter displaying narrow zones of thinning (![Formula][1] ) and negligible taper. Three of the intervals switch geometry around the diapir. The CHS-bounding unconformities typically intersect the salt at cusps and are continuous with bright amplitudes in the minibasins that tie to biostratigraphically defined condensed sections. CHS represent 50–500 kyr time spans and correlate well with fourth-order sea-level cycles. We corroborated many aspects of the published model of CHS development, showed that the formation of CHS due to drape folding was independent of depositional environment and related to fluctuations in sea level and sediment input. The style of CHS is generally determined by the interplay between salt-rise and sediment-accumulation rates, but variable CHS geometries around the diapir within the same interval suggested that the ultimate control is the roof thickness. Our results are critical to understanding and predicting aspects of hydrocarbon traps against salt, including trap geometry, and reservoir distribution. [1]: /embed/mml-math-1.gif

This paper illustrates the strengths and limitations of a novel “sub-basin-scale” seismic attribute (van Hoek et al. in this special section). It focuses on the use of the unconformity attribute within exploration and appraisal work flows as a “regional-seal indicator” and its applications in de-risking prospects in deepwater clastics depositional environments. Integration of this new attribute with traditional data and interpretation techniques is discussed in three case studies (Figure 1)

This paper describes how BP and its partners successfully acquired 450 km2 of 3D seismic data in a complex area of the Llanos Foothills in Colombia. The data will be valuable for future decisions regarding the development of the Florena, Pauto, and Volcanera fields. The work area is in the Piedemonte Llanero, in the foothills of the Cordillera Oriental of Colombia. The stratigraphy involves Paleozoic, Cretaceous, and Tertiary rocks, and multiple regional unconformities. The main hydrocarbon accumulations are in the sandy levels of the Guadalupe Group and Mirador and Barco formations. The structural complexity of the foothills increases significantly from south to north, from relatively simple structures in Cusiana to tight asymmetric structures with higher dips in Cupiagua and Cupiagua South. Complexity continues to increase toward the north, where the shortening is greater as evidenced by surface geology (Figure 1).

Unconformity-type deposits supply a significant amount of the world’s uranium and consist of uranium that is generally codeposited with graphite in a fault zone. The low resistivity of the graphite produces a significant contrast in electrical resistivity, which can be located with electromagnetic (EM) methods. The Athabasca Basin in Western Canada hosts significant uranium deposits, and exploration in deeper parts of the basin has required the application of new EM methods. This paper presents an evaluation of the audiomagnetotelluric (AMT) exploration method at the McArthur River mine in the Athabasca Basin. AMT data were collected at 132 stations on a grid, and two-dimensional (2D) and three-dimensional (3D) inversions were used to generate resistivity models. These models showed two major results: (1) a significant conductor coincident with a major basement fault (P2) and the uranium deposits (this conductor begins at the unconformity at a depth of [Formula: see text] and extends to a depth of at least three km) and (2) a resistive halo which might be caused by the silicification associated with mineralization. However, synthetic inversions showed that this halo could be an artifact of smoothing function in the inversion scheme. The 2D inversions were validated by synthetic inversions, comparison with the 3D inversion models, and correlation with well-log information. 3D AMT forward modeling showed that strong 3D effects are not present in the AMT impedance data. Induction vectors showed more evidence of complexity, but the inclusion of these data in the inversion improved subsurface resolution.

Several authors have changed employment since this paper was written. Those authors and their new locations are: David Dolberg, Mobil Oil Canada, Calgary, Alberta, Canada; Jan Helgesen, CGG Norge, Hovik, Norway; Tore Hakon Hanssen, Fortum Petroleum, Oslo, Norway; Ingrid Magnus, Norsk Hydro ASA, Bergen, Norway; Girish Saigal, Norsk Hydro ASA, Oslo, Norway; and Bengt K. Pedersen, Norwegian University of Science and Technology, Trondheim, Norway. Lavrans Field lies offshore Norway on the western extreme of Halten Terrace, 15 km south of Smorbukk Field (Figure 1). Exploration in this area has been extremely active, yielding two large gas and condensate discoveries within Petroleum License 199—Lavrans Field to the east and Kristin Field to the west. Combined reserves are approximately 1200 million barrels oil equivalent of gas and condensate. Unique conditions have come together to provide hydrocarbons and preserved porosity at depths greater than 5 km. Figure 1. Lavrans and Kristin fields relative to Smor-bukk. Field outlines (rose) are drawn on the map of the Base Cretaceous Unconformity. View is to the north. Hydrocarbon-bearing sandstones at Lavrans have a thickness of 600 m. These reservoirs consist of shallow-marine deposits of Jurassic age. Although the facies can be laterally extensive, the overprint of diagenesis makes prediction of reservoir quality difficult. Seismic inversion provides insight to porosity variations away from limited well control. Thus, seismic inversion can be a valuable tool for reservoir characterization prior to field development. This study developed out of a need to better understand the significance of seismic amplitude variations over the crest of Lavrans Field (Figure 2). Hypotheses examined to explain amplitude variations were fluid effects, porosity, pressure changes, or processing and illumination artifacts. Figure 2. Cross-line 3185 is a dip line through well 6406/2-2. Seismic anomalies are seen below the Top Ile Formation. Understanding the meaning of these amplitudes was a primary motivation …

Summary A seismic Wheeler Volume is an important tool for seismic stratigraphic interpretation to portray the geologic timespace distribution of strata and the related depositional, non-depositional and erosional events. It is also valuable for seismic geomorphological interpretation by facilitating the extraction of isochronal stratal slices. In complicated geologic settings (e.g., faults and unconformities), generation of a reasonable seismic Wheeler volume is difficult for the current seismic Wheeler volume generation methods. In this paper, we present a method to convert a seismic data volume from travel time-space domain into Wheeler domain via its corresponding a relative geologic time volume generated by phase unwrapping. Tests by model and real seismic data suggest that our method can effectively deal with complicated geological situations with faulting, folding and unconformities, resulting in a reasonable seismic Wheeler volume.

Reservoir compartmentalization of the Dollarhide Field has to be understood in order to optimize recovery of remaining reserves. Porosity and permeability heterogeneity is a function of the interaction of stratigraphic and structural elements such as fractures, faults, subcrops and channels. Diagenesis and dissolution play an important role on the heterogeneity of the reservoir. Some of the reservoir-scale features are small and beyond the resolution of conventional seismic. Research is being conducted on the use of 3D seismic attributes to better delineate reservoir parameters important for fluid flow models. This paper presents our current understanding of the lineaments of the field after using some computed 3D seismic attributes. Coherence with two passes of edgepreserving smoothing has been a helpful attribute. The Dollarhide Field is a north-south trending asymmetric anticline bound by major reverse faults on its east flank. The anticline is associated with a northeast striking, rightlateral, strike-slip fault, and it is cut by an en-echelon system of northeast striking, right lateral, synthetic, Riedelshear faults. Conjugate, northwest striking, antithetic shear faults have been identified on the downthrown block of the field. Some lineaments in the upthrown block with the same trend could have the same origin. The structure was formed during the Late Paleozoic orogeny as the result of the collision of North and South American plates, when the Central Basin Platform was uplifted. The structure is truncated at its crest by the unconformity at the base of Early Permian carbonates. Subcropping belts of carbonates, cherts, and shales show the configuration of the structure before Early Permian times. Reactivation of pre-existing faults may have occurred during Early Tertiary Laramide orogeny. When compared with porosity-thickness maps of the reservoirs, the subcrop pattern and conjugate shear faults show a strong correlation with porosity trend segmentation.

Summary We report measurements on glauconitic sandstones as a function of pressure. Glauconite is an iron rich variety of clay that can be found as individual pellets, composite grains, and intergranular cement. Identifying glauconite in the subsurface is important for depositional environment interpretation, stratigraphic correlation, dating, tracing of unconformities, geochemical exploration in marine environments, and reservoir quality prediction in glauconitic sandstones. In order to identify, track, and characterize glauconitic sandstones from seismic data, it is important to relate the elastic properties of glauconitic sandstone to their physical properties, most important among which are mineralogy, porosity, and rock texture. To this end, this paper presents laboratory measurements of ultrasonic velocities, porosity and permeability of glauconitic sandstones from the Caballos Formation, Putumayo Basin (Colombia). To differentiate between glauconite bearing and non-glauconite bearing rocks, we compare their physical and acoustic properties along with those of a pure glauconite sample. Based on laboratory data, we have established that rocks containing glauconite have at the same porosity smaller permeability and smaller velocity than sands without glauconite. Probably the most exciting and practically meaningful result is that the quality of reservoir rock (specifically, permeability) can be discriminated not only by porosity but also by Poisson’s ratio (or Vp/Vs ratio). Specifically, at the same porosity, the samples with lower permeability have higher Poisson’s ratio. In other words, our rock-physics diagnostic of Caballos Formation shows that we can determine porosity and permeability from a combination of P- and S-wave data.

Accurate near surface velocity models can be derived from refraction arrivals on conventional field records (shot for reflection data) in complex overthrust areas. This is true even when the reflection image is poor and reflection based interval velocities are not practical. Refractions are easiest to interpret when the refracting surface is a buried erosional unconformity with overthrust layers subcropping along the unconformity. Velocity accuracy is about 10%. Velocity resolution is about 2%. Spacial resolution for locating the boundaries between velocity units is about 3 group locations. Velocities can be measured when the lateral extent of the formation along the refracting surface is as short as 500 m. Refractions are more difficult to interpret where thrusted units outcrop at the surface. In this case, the refracting surfaces are often the transitions between weathered, less weathered and non-weathered rock of the same formation. Velocity resolution is still about 5% but the spacial resolution is less than above. This paper presents refraction velocity models from both types of terrain, confirmed by well data obtained after the refraction analysis was completed.

Seismic data acquired in the central Mediterranean has been difficult to effectively process and interpret for many years. These difficulties are a direct result of the complex geology of the area, and exacerbated by a hard water bottom and a highly reflective Messinian unconformity directly beneath it which excite strong long period surface and interbed multiples. Complex series of out-ofthe-plane diffractions and back-scattered head waves result from the rugose, karsted nature of both the Messinian and Cretaceous unconformities. The intermediate depth Mesozoic section has a strong three-dimensional component, having undergone wrench fault tectonism and probable diagenesis. These complications are superposed on the low seismic signal reflectivity representative of carbonate on carbonate sequences. We describe in this paper alternative approaches using modern seismic processing techniques that attempt to generate a superior 2D image in the presence of 3D noise trains.

The vast and growing literature on the search for stratigraphic traps for petroleum ignores gravity gradients, for which the theory has been available since the heyday of the Eötvös torsion balance decades ago. These are discussed in this paper. The horizontal and vertical gradients can be measured with available gravimeters and (to a limited extent) with the Eötvös torsion balance. A major advantage of the gradient method is that surveying to determine position and elevation of the station is not required. Both theory and practice have been reported in the geophysical literature, but the important application to stratigraphic traps has not been mentioned. We evaluate here the method for locating “pinchouts”, a term which embraces “stratigraphic” and “unconformity” traps in Halbouty’s (1972) classification. Both position and depth of the assumed pinchout are determined by the gradient anomaly. The magnitudes of anomalies of horizontal and vertical gradients are about equal. However, pending new instrumentation, only the horizontal gradient is practically useful for field surveys. The gradient method is quantitatively promising and, used in conjunction with other methods, should significantly advance the search for stratigraphic traps for petroleum.

The eroded Mississippian surface is the major unconformity in the province of Alberta. To map its erosional highs and lows is most important, because the Mississippian may be productive of hydrocarbons or may cloak the attitude of deeper sediments from which production is sought. This paper deals with the methods of presentation of reflection seismic data to that end, together with a suggested recording instrument technique. Some of the interpretive problems, and the possible significance of Mississippian porosity on the acoustic impedance of its reflection are mentioned. Maps of similar data, one geological and the other reflection seismic, are presented for comparison. The former is obtained from drilled wells and the latter from reflection shooting performed prior to drilling.

With recent advances in seismic processing and display capabilities, much emphasis has been placed on stratigraphic interpretation of seismic data. The shift from its structural counterpart began in the early ’70s when Peter Vail of Exxon presented several classic papers on seismic stratigraphy. (An excellent summary of this work can be found in AAPG Memoir 26, 1977). The original emphasis of seismic stratigraphic interpretation was on reflection‐character classifications and the identification of unconformities. This approach gained a large following during the mid‐70s. It was during this period that many geologists began to work more closely with seismic data. New interpretation techniques have greatly increased our understanding of depositional environments and lateral facies changes within underexplored portions of sedimentary basins. Fortunately, the advances in seismic stratigraphy have reduced the number of unsuccessful wildcat drilling locations. However, dry holes are still being drilled.