Xavier Janson

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

Three-dimensional geological and synthetic seismic model of Early Permian redeposited basinal carbonate deposits, Victorio Canyon, west Texas

The Lower Permian outcrops of Victorio Canyon, in the Sierra Diablo Mountains in west Texas, show undisturbed stratigraphy of carbonate toe-of-slope and basinal deposits. These rocks consist of a vertical stack of carbonate debris-flow deposits and hyperconcentrated density-flow deposits, thick skeletal-ooid grainstone deposited as basin-floor fans, various reworked muddy carbonate deposits, and fine siliciclastic siltstones. This succession was deposited within five sequences that include the upper Hueco Formation (late Wolfcampian) through the Bone Spring Formation (middle Leonardian). Using a ground-based light detecting and ranging-generated high-resolution digital outcrop model (DOM) aged as a template, we mapped and digitized the stratigraphy of the toe-of-slope and basinal deposits on the 5-cm (2-in.) precision DOM. On the basis of the digitized stratigraphic contacts, several regional surfaces were constructed, and a 3-D geocellular model was built. Facies information within this model is extrapolated from measured section data using both a kriging algorithm and stochastic simulations. Using impedance values extracted from a subsurface analog, a 3-D impedance model was created for both the kriged and the stochastic models. Both models incorporate fine-scale stratigraphic architecture. In addition, the stochastic impedance model incorporates spatially correlated noise, resulting in more realistic synthetic seismograms. Three-dimensional synthetic seismograms were calculated at 20, 40, and 80 Hz. The reservoir-prone facies is skeletal-ooid grainstone deposited as a 1.5-km 750-m (0.93-mi 2460-ft) basin-floor fan up to 15 m (49 ft) thick. This basin-floor fan is subtly imaged in vertical seismic section at a frequency below 80 Hz. It is, however, better recognized on time slices with peak frequency as low as 20 Hz and even better delineated on horizon slices that parallel the stratigraphy.

Characterizing seismic bright spots in deeply buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, Western China

Anomalous seismic-amplitude bright spots are a common feature in deeply buried (5500 to 6500 m) Ordovician limestone strata in the Central Tabei Uplift area of the Tarim Basin in northwest China. Those anomalies have proven to be useful indicators of reservoir quality. The bright spots as seen on seismic data are tied to high-gamma ray, low-velocity zones in wireline logs, and correspond to clastic cave sediment-fills in the host limestone in core. Synthetic seismic models confirm this relationship between seismic bright spots and cave-sediment fills. A seismic traveltime map of the top Ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. A mature surface drainage system interacted with a near-surface karst system and allowed terrigenous sediments to enter an underground cave system. Karst-related bright spots probably correspo...

Controlling Parameters on Facies Geometries of the Bahamas, an Isolated Carbonate Platform Environment

The Bahamas are among the most extensively studied carbonate regions in the world, and a number of phenomena typical of calcareous environments have been first observed in the Bahamas. Early geological research in the Bahamas was undertaken by Nelson (1853B) who surveyed their geography and topography. He noticed the “remarkable lowness of profile” and the dynamics of construction and destruction of the islands, outlined the biota and lithologies, described the formation of the carbonate rocks, and noticed the eolian origin of many Bahamian islands. Forty years later, the examination of modern carbonate environments rapidly progressed with the expedition of L. and A. Agassiz in 1893 (Agassiz 1894). Their explorations focused mainly on the fringing reefs of GE Great Bahama Bank. Research on abiotic carbonate components followed, by Vaughan (1914) who emphasized that carbonate constituents can originate from both skeletal secretion and chemical precipitation, and introduced the terms “organic” and “inorganic” limestones. Black (1933) first characterized the sedimentary facies on Great Bahama Bank and noted the significance of the widespread aragonitic mud. The sand-sized calcareous components of the Bahamas and their origin, including ooid sands, were described in detail in the classic papers by Illing (1954) and Newell et al. (1960).

Seismic architecture of a Lower Cretaceous platform-to-slope system, Santa Agueda and Poza Rica fields, Mexico

Two three-dimensional seismic data sets over the Albian western Golden Lane margin and time-equivalent basinal deposits of Poza Rica field allowed us to investigate the linked architecture of a steep-sided carbonate platform (El Abra Formation) and a thick accumulation of redeposited carbonate sediment at the toe of the slope and in the basinal area (Tamabra Formation). Regional seismic cross sections show that the most aggrading Albian platform has an eroded platform top, a scalloped margin, and a channelized slope that are equivalent to a 20-km (12.4-mi)-wide, westward-thinning, thick toe-of-slope apron made of chaotic, contorted, mounded, moderate- to high-amplitude reflections. Detailed reflection geometries in the Albian toe-of-slope and basinal deposits consist of chaotic to short, discontinuous, low-amplitude reflection at the toe of the slope of the Golden Lane platform, laterally changing to a discontinuous mounded, shingling reflection, which ultimately turns into high-amplitude parallel reflections. We interpret this lateral change to reflect the seismic signature of the change from the block- and debris-flow–dominated toe-of-slope area, to debris-flow and concentrated density flow deposits in the basin that ultimately grade laterally into pelagic deposits. On a flattened seismic slice, mounded reflections correspond to lobate to fan-shaped seismic events several kilometers wide that are interpreted as a carbonate basin-floor fan. Comparison between core and seismic data shows a dominance of debris flows in the lower two Albian sequences (Albian 1 and Albian 2) that grade vertically into more lobate concentrated density flows and turbidites in the upper two Albian sequences (Albian 3 and Albian 4). Seismic data used in this study, combined with core observations, do not support the interpretation of the Albian Tamabra Formation being of shallow-water origin. Seismic features identified as basin-floor fan, channel, and debris-flow deposits have a shape and size that are similar to those of other redeposited basinal carbonate deposits elsewhere. The seismic architecture shows that the Poza Rica field is a typical example of thick accumulation of grainy porous carbonate deposits in a basinal setting. This example shows the potential of a large hydrocarbon accumulation in a tectonically modified stratigraphic trap around shallow-water carbonate platforms.

Architecture and facies differentiation within a Middle Miocene carbonate platform, Ermenek, Mut Basin, southern Turkey

Abstract In south-central Turkey, a carbonate platform system of early middle Miocene age is exposed in three-dimensional outcrops displaying a rich variety of carbonate facies associated with exceptionally well-preserved depositional geometries. This paper presents a detailed reconstruction of the geometries and facies organization across the prograding margin of one intra-platform carbonate bank that grew during the Langhian on the Ermenek Platform. The total thickness of the margin is approximately 250 m, and it has prograded over a distance of 1.2 km. The geometrical pattern shows an alternation between sigmoid, sigmoid-oblique, and oblique accretionary units at different scales. Based on the facies distribution and the geometrical framework two large-scale depositional sequences and eight medium-scale depositional sequences were defined. The general evolution from a low-angle shelf geometry to a prograding flat-topped platform was associated with an evolution from oligophotic-dominated carbonate producers, such as large benthic foraminifera, molluscs, echinoderms, red algae and bryozoans at the base, to mesophotic and euphotic carbonate producer organisms, such as corals, red algae and porcellaneous small benthic foraminifera at the top. The eight medium cycles were defined primarily using the depositional geometries, since facies changes were observed only locally within these cycles. Several mechanisms influenced the stratigraphic architecture of this margin: (1) eustatic sea-level controlled the overall transgressive-regressive Langhian sequence, and two superposed large-scale sequences. Medium cycles were probably also influenced by higher frequency sea-level fluctuations; (b) climate change probably influenced the overall evolution of the faunal assemblage; and (c) antecedent topography determined the overall architecture of a shelf bordering a deeper basin.

The Gulf: Facies Belts, Physical, Chemical, and Biological Parameters of Sedimentation on a Carbonate Ramp

The Holocene of The Gulf, also referred to as the Arabian or Persian Gulf, is frequently cited as a classic example of a mixed carbonate-siliciclastic ramp system for an arid climate. This notion of a ramp is supported by the recognition that The Gulf area has a dominant shallow water carbonate/evaporite basin fill from the Permian to today despite a complex tectonic history (Alsharhan and Kendall 2003). The current depositional setting is that of a proximal foreland ramp (Burchette and Wright 1992; Evans 1995; Kirkham 1998). Walkden and Williams (1998), however, argue that since The Gulf has been above sea level for over much of the past 2.5 Ma, and since it is in tectonic, eustatic and depositional disequilibrium it should not be considered a ramp. Despite this controversy, the Holocene sedimentary fill of the current Gulf has been and will continue to be used as a model for a carbonate ramp. This interest in the area is hightened by the fact that is one of the few places in which Holocene dolomite and evaporites form.

Seismic expressions of a Miocene prograding carbonate margin, Mut Basin, Turkey

The detailed stratigraphic architecture of a Miocene intraplatform prograding carbonate margin, outcropping in Mut Basin, Turkey, was used to build two-dimensional synthetic seismograms by normal ray-tracing and finite-difference, exploding-reflector methods. The synthetic seismic models at various frequencies display the overall prograding architecture of the 1.5-km (0.95-mi) 250-m (820-ft) prograding margin, even at 40-Hz frequency; however, most of the complex stratigraphic architecture is below standard seismic resolution. Sigmoidal outcrop geometries create complex, shingled, seismic reflections that contain pseudodownlap, pseudotoplap, and various truncations at 40-Hz frequency. In addition, reflections that do not correspond to any impedance contrast are generated by a tuning effect within the clinoform. Interference patterns created by the complex impedance distribution create reflections that change phase laterally and cross stratigraphic timelines. Several high-impedance coral buildups at the slope break and on the platform top cause reflections to change phase and amplitude along the timeline. In addition, lateral-impedance variations generate mounded reflections that, in places, cross timelines and resemble seismic images for buildups or anticlinal structures.Several of these resolution problems and interference patterns are reduced by an increase of the frequency. For example, at 80 Hz peak frequency, the sigmoidal geometries are better imaged. Improved resolution was also achieved on the synthetic seismic data simply by dividing trace spacing from 25 to 12.5 m (82 to 39 ft) but keeping the modeling frequency constant, indicating that for stratigraphic interpretation, horizontal seismic resolution is at least as important as vertical seismic resolution. This implies that adjusting the acquisition geometries would improve the seismic resolution without compromising depth by increased frequency. For seismic interpretation, this study shows how little of a complex stratigraphic architecture is truly imaged on seismic data and how complex impedance distribution can create false seismic reflection geometries that impede a correct seismic facies and sequence-stratigraphic interpretation.

Carbonate reservoir characterization using seismic diffraction imaging

Although extremely prolific worldwide, carbonate reservoirs are challenging to characterize using traditional seismic reflection imaging techniques. We used computational experiments with synthetic models to demonstrate the possibility that seismic diffraction imaging has of overcoming common obstacles associated with seismic reflection imaging and aiding interpreters of carbonate systems. Diffraction imaging improved the horizontal resolution of individual voids in a karst reservoir model and identification of heterogeneous regions below the resolution of reflections in a reservoir-scale model.

An Ultra-Deep Paleokarst System In the Ordovician, North-Central Tarim Basin, China: Outcrop Analog And Synthetic Seismic Models

Summary Exposed Ordovician strata in the western Tarim Basin in Northeastern China show a karst system that consists of karst towers and paleocave breccias. These outcrops are used to build 3D geocellular model and 3D synthetic seismogram. The low impedance paleocave breccias create distinctive high amplitude seismic anomalies below the unconformity reflection. A larger model is built with the top Ordovician surface mapped in the subsurface and with randomly distributed ellipses to simulate the paleocaves. The resulting seismograms show realistic reflection character and geometries. Impedance model with 150 m to 250 m wide by at least 20m thick paleocaves with a 175 % impedance contrast between the paleocave and host rock results in 3D synthetic seismogram that mimic the amplitude anomalies observed in the subsurface data.