Mobile Shale Research Articles

3D Seismic study of a shale expulsion syncline at the base of the Champion delta, offshore Brunei and its implications for the early structural evolution of large delta systems

An example of a Late Miocene expulsion rollover syncline related to mobile shale is described from 3D seismic data offshore Brunei. Prodelta shales accumulated in the subsiding syncline were altered after deep burial by overpressure increase and hydraulic fracturing. Consequently seismic facies was modified from a continuous divergent reflection pattern to a low-amplitude, disrupted to chaotic seismic facies bounded by a high-amplitude event that cross-cuts original stratification. Without the resolution provided by 3D data, the high-amplitude event would be mistaken for early growth faults and the subdued country-rock reflections for mobile shale, typical features at the base of large deltas. This study shows the existence of expulsion rollovers related to mobile shale, which have not been described before, and demonstrates that not all bright reflections at chaotic-coherent facies boundaries are actually fault planes. It also suggests that expulsion synclines, even during early phases of delta development may be compaction-related features rather than ones that drive the shale mass out of the area of progradation.

Chronostratigraphy and tectonostratigraphy of the Columbus Basin, eastern offshore Trinidad

The Columbus Basin, forming the easternmost part of the Eastern Venezuela Basin, is situated along the obliquely converging margins of the Caribbean and South American plates. The two primary structural elements that characterize the basin are (1) transpressional northeast-southwest-trending anticlines and (2) northwest-southeast-oriented, down-to-the-northeast, extension normal faults. The basin was filled throughout the Pliocene and Pleistocene by more than 40,000 ft (>12,200 m) of clastic sediment supplied primarily by the Paleo-Orinoco Delta system. The delta prograded eastward over a storm-influenced and current-influenced shelf during the Pliocene-Pleistocene, depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower Pliocene to pre-Pliocene mobile shale facies. Biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two-dimensional and three-dimensional seismic data to construct a chronostratigraphic framework for the basin. As a result, several observations were made regarding the basin's geology that have a bearing on exploration risk and success: (1) megasequences wedge bidirectionally; (2) consideration of hydrocarbon-system risk across any area requires looking at these sequences as complete paleofeatures; (3) reservoir location is influenced by structural elements in the basin; (4) the lower limit of a good-quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location; (5) reservoir quality of deep-marine strata is strongly influenced by both the type of shelf system developed (bypass or aggradational) and the location of both subaerial and submarine highs; and (6) submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure, migration, and trapping of hydrocarbons and the distribution of hydrocarbon type. (Begin page 1906)

Comparison of gravity‐driven deformation styles and behavior associated with mobile shales and salt

Structures produced by salt and mobile shales are commonly similar; however, the material behavior of the two is different. Salt mobility is a fundamental material property, shale mobility only occurs if overpressured fluids are present. Dewatering of shales will stop their mobility, while renewed burial or the onset of an internal overpressuring process (e.g., diagenetic release of water or hydrocarbon generation) may renew mobility. Consequently, structures in mobile salt will envolve continuously until salt withdrawal produces touchdowns sufficient to stop salt mobility. Shale mobility may follow a deformation sequence similar to salt, or it may display a more episodic evolution reflecting critical overpressuring events. While salt mobility is confined to specific lithological units, the same is not the case for shale. The overpressuring of shales is strongly dependent on depth, so that mobile shale zones may cut across time/bedding boundaries. The differences in mechanical behavior lead to differences in structural style although many basic aspects of gravity tectonics remain the same. (1) Prekinematic structures and synkinematic deformation occur in both salt and shale tectonics. However, much of the prekinematic deformation in mobile shale‐dominated deltas may be lost by burial and conversion of the prekinematic sequence into mobile shales. (2) Fault‐dominated depocenters occur in salt and shale tectonics. In the Niger delta, overpressured shales vary in thickness from thin decollement zones to massive chaotic zones some 4–6 km thick. Fault‐controlled basins develop over mobile shales. Maximum basin depth is approximately the thickness of the mobile shale plus the thickness of the overlying pregrowth fault strata (i.e., some 4–8 km). (3) Diapirs are common to both salt and shale tectonics. Normal faults associated with reactive diapirism are common in both. Salt has the potential to almost completely evacuate a particular volume, resulting in local touchdowns or welds. Shale may also create touchdown areas, but complete collapse is uncommon because a large volume of immobile dewatered shale is usually left behind. (4) Salt nappes can cover extensive areas in the Gulf of Mexico. Some limited shale tongues occur in the Niger delta. They form imbricate thrusts that pass down dip into gravity flows and slumped blocks of shale derived from the exposed mobile shale.

A critique of techniques for modelling normal-fault and rollover geometries

Abstract A review of the published literature indicates that antithetic oblique shear, at an angle of 30 ± 10° measured from the vertical, is the most appropriate algorithm for quantitative modelling of geometric relationships between downward-flattening normal faults and the strata in their hanging walls. This conclusion is based on 2D analysis of 11 natural examples (listric normal faults without apparent salt involvement, shale mobility, or footwall deformation) and selected laboratory physical models. It may not apply where mobile salt, mobile shale, or significant footwall tilting or deformation is present. Antithetic oblique shear, like all modelling algorithms, does not accurately simulate natural deformation processes, which are significantly more complex.

Petroleum Geology of the Deltaic Sequence, Rio Del Rey Basin, Offshore Cameroon

The Rio Del Rey Basin of offshore Cameroon comprises the easternmost portion of the Niger delta complex. In the delta flank setting, the overall package is thinner and stratigraphic correlation simpler than in the depocenter to the west so the distribution of reservoirs and seals is well defined. Productive reservoirs are shallow, typically less than 2000 m, and mostly hydropressured to slightly overpressured. Reservoir properties generally are excellent and seismic bright-spot technology has played a major role in exploration and development efforts. Rio Del Rey is characterized by three main structural styles from north to south: (1) detachment-based growth faults, (2) highly faulted, mobile shale cored domes and ridges, and (3) toe thrusts and folds. Virtually all individual traps are fault dependent and lack of sufficient internal seals in sand-rich wave-dominated deltaic sequences can limit the objective window. Most fields are downthrown fault traps in which hanging-wall reservoirs are effectively sealed against older prodelta and marine shales. All large oil fields are located in the dome and ridge province and are potential analogs for similar structures now being explored on the upper slope off Nigeria. The best quality source rocks identified thus far are Paleocene to Eocene marine shales. Oilmore » vs. gas distribution in the basin is a function of variations in the kerogen type, organic richness, and maturity of these source rocks. A significant quantity of the gas found to date in Rio Del Rey is biogenic in origin. The Cameroon charge model may aid in better understanding hydrocarbon distribution on offshore Nigeria, where the Miocene deltaic section is much thicker and potential Paleocene-Eocene source rocks likely are unpenetrated. The shallow deltaic play in Cameroon is relatively mature. Future exploration likely will focus on identifying deeper, nonbright-spot-supported opportunities including potential stratigraphic traps.« less

Deltaic Faulting and Subsidence: Analog Modeling

Scaled experiments with sand layers overlying viscous silicone putty have been used to investigate the behavior of deltaic sediments prograding over salt or mobile shales. Differential loading caused by a sand wedge prograding over a viscous putty layer induces a forward expulsion of the viscous material. This causes the putty to thin beneath the sand wedge and to thicken at the wedge toe. It results in extension and subsidence in the sand wedge. The predominant dip of the extensional faults is in the progradation direction except in the toe bulge area where a major fault may occur with an opposite (counter regional dip). The experiments examined how changes in model parameters affect the resultant fault geometries: Increasing the putty thickness leads to increase in the amount of extension and degree of block rotation, both of which decrease upwards into younger sediments; a sloping basement/putty interface leads to a significant increase in the extension of the sand wedge; fast progradation rates lead to widely spaced faulting whereas slow progradation rates lead to closely spaced faulting; basement fault steps, associated with changes in viscous layer thickness, are also demonstrated to locate and locally reorient faults in the overlying sand wedge. These conceptsmore » can assist the interpreter in defining the shape of faulted traps, particularly at depth or where seismic quality deteriorates, and in understanding the evolution and timing of trap formation.« less

Geometric Constraints Derived from the Law of Conservation of Volume and Applied to Evolutionary Models for Detachment Folding: Geologic Note: (1)

This paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones. The common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel. Limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane. The basic depth to detachment calculation, which derives from the Law of Conservation of Volume, requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment. This imposes a geometric constraint with whi h a constant limb length anticline, growing by increasing limb dip above a fixed detachment plane, cannot comply unless, in the early stages, mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then, in the later stages, the flow direction reverses. Relatively few detachment zones contain a significant amount of mobile salt or shale. Therefore, in most concentric detachment fold trains, a basic geometric incompatibility exists between the conventional geometric model and the Law of Conservation of Volume. The conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows. In this model, the anticlinal axes generally are fixed and the synclinal axes active, which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons.

Structural Style and Petroleum Prospects of the Kuqa Depression, Northern Tarim Basin, Northwest China: ABSTRACT

The Kuqa depression is a 400-km {times} 100-km, east-west-trending foredeep basin located on the northern margin of the Tarim craton in northwestern China. A 10-km-thick nonmarine Mesozoic and Cenozoic sedimentary section has accumulated in the basin as the Tian Shan range to the north and has been episodically uplifted and thrust southward, tectonically loading the craton margin and providing source areas for clastic sediments. Regional mapping from Landsat images and field reconnaissance reveal important details of the structural style of the Kuqa depression. The Kuqa depression is characterized by thin-skinned deformation resulting in a series of steep, faulted, elongate folds. Thick, mobile Tertiary shale sequences and an Upper Cretaceous-lower Tertiary evaporite section in the west form detachment horizons separating disharmonically deformed structural packages. At the southern margin of the depression, the Mesozoic-Cenozoic section overrides faulted Paleozoic strata of the North Tarim uplift. Here, the Neogene Qiulitage anticline extends 250 km along the basin margin. Beginning in 1984, several important oil discoveries have been made in the Paleozoic section on the North Tarim uplift. Analysis of an oil sample indicates its compatibility with a lower Paleozoic marine source. Other potential source rocks in the Kuqa depression include Jurassic oil shales andmore » coals. Sandstone reservoir strata and structural trapping geometries are numerous, but exploration risks are complicated by the presence of the ductile evaporite and shale seals. If nonmarine source rocks are also present in the thick Cenozoic section, the Kuqa depression may be an attractive exploration target.« less