How do tectonics influence the initiation and evolution of submarine canyons A case study from the Otway Basin, SE Australia

The architecture of canyon-fills can provide a valuable record of the link between tectonics, sedimentation, and depositional processes in submarine settings. We integrate 3D and 2D seismic reflection data to investigate the dominant tectonics and sedimentary processes involved in the formation of two deeply buried (c. 500 m below seafloor), and large (c. 3-6 km wide, >35 km long) Late Miocene submarine canyons. We found the plate tectonic-scale events (i.e. continental breakup and shortening) have a first-order influence on the submarine canyon initiation and evolution. Initially, the Late Cretaceous (c. 65 Ma) separation of Australia and Antarctica resulted in extensional fault systems, which then formed stair-shaped paleo-seabed. This inherited seabed topography allowed gravity-driven processes (i.e. turbidity currents and mass-transport complexes) to occur. Subsequently, the Late Miocene (c. 5 Ma) collision of Australia and Eurasia, and the resulting uplift and exhumation, have resulted in a prominent unconformity surface that coincides with the base of the canyons. We suggest that the Late Miocene intensive tectonics and associated seismicity have resulted in instability in the upper slope that consequently gave rise to emplacement of MTCs, initiating the canyons formation. Therefore, we indicate that regional tectonics play a key role in the initiation and development of submarine canyons.

Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.

ГЕОЛОГИЧЕСКИЕ ПРЕДПОСЫЛКИ ПОИСКОВ ЗАЛЕЖЕЙ НЕФТИ И ГАЗА В СТРАТИГРАФИЧЕСКИХ И ЛИТОЛОГИЧЕСКИХ ЛОВУШКАХ ЮГО-ЗАПАДНОГО БОРТА НИЖНЕКУРИНСКОЙ ВПАДИНЫ

The Lower Kura depression is a recognized oil and gas generating basin, characterized by positive stratigraphic, lithological-facies and structural-tectonic criteria for oil and gas content. However, it is necessary to additionally assess the prospects for oil and gas content of the southwestern side of the basin, guided by the criteria for the presence of reservoirs, seals and traps, paleotectonic criteria and seismic geological indicators.According to the results obtained from a detailed study by geophysical methods of the Kyurovdag-Neftchala belt and the territories framed to it, it was found that developed lithologically limited and stratigraphic traps in the Sarkhanbeyli, Orta Mugan, Shargi Shorsulu and Babazanan areas have all the signs of oil and gas prospects. These traps are located at a depth of no more than 4.5 km between the Mesozoic paleorelief protrusions and the Pleocene sediments covering them. It was the disagreement between these rocks that played an important role in the migration and accumulation of hydrocarbons. It should be noted that the traps formed in the process of sedimentation by primary reservoirs above the unconformity surfaces are sedimentation-stratigraphic and adjacent to the unconformity surface. Studies have revealed similar traps in deeper pinching horizons, which can be considered promising in terms of the development of reservoirs, cap rocks and oil-damaging reservoirs.

Analysis of vertical position relationships between igneous sills and an unconformity surface — Interpretation of seismic profiles from the Northern Tarim Basin, NW China

Sill emplacement mechanisms are very complex, diverse, and regional, and insights from sill reflections are helpful for understanding the emplacement process of magma in the Tarim Basin. This study takes advantage of high-quality 2D seismic data, which are rarely used to study sills in the Tarim Basin, to analyze the sills’ geometric characteristics, plan-view distributions, emplacement timing, and emplacement mechanisms with unconformity surfaces. In the seismic-reflection profiles of the middle-upper Ordovician in the North depression and the southern part of the Tabei uplift in the Tarim Basin, sills with strong positive polarity reflections appear, and they are closely distributed near the Tg52 unconformity surface, which represents the interface between Middle Ordovician limestone and Upper Ordovician mudstone. According to the vertical position of the sills relative to the unconformity, we can divide the sills into saucer-shaped or quasi-saucer-shaped sills above the unconformity surface, sill complexes and saucer-shaped sills on the unconformity surface, and saucer-shaped sills below the unconformity surface. Potential hydrothermal vents and peripheral faults associated with sill intrusion terminate upward in the Middle Permian strata, suggesting that these sills formed in the Middle Permian. Sills with inner flat sheets on the Tg52 unconformity surface formed when the magma ascended and encountered an abrupt change in the fracture toughness and tensile strength between the two adjacent host rock layers. The sills above and on the Tg52 unconformity surface overlap or are vertically linked; therefore, the sills above the Tg52 unconformity surface are the result of the continuous upward expansion of the sills on the unconformity surface, forming sill complexes. Our findings further confirm that unconformities are important interfaces that affect the emplacement of sills.

The significance of karst unconformities on overlying resource shales: Lessons learned from the Devonian Woodford Formation applied to the Permian Wolfcamp Shale

We have summarized the threefold significance of karst unconformity boundary: (1) The development of a sequence stratigraphic model for the Devonian Woodford Shale Formation is transferable to the Upper Wolfcamp in the Permian Basin, (2) demonstration of the more general application of that model beyond the Woodford to other resource shales, and (3) illustration of a modification of common sequence stratigraphy models specifically to unconventional resource shales. During early transgression, marine encroachment into the paleolows created anoxic, hypersaline marine “pockets” conducive to the preservation of organic matter. The result is deposition of thick, laterally discontinuous, organic-rich strata stratigraphically at or near the unconformity surface. This pattern of deposition and distribution of the organic-rich shale has been well-documented in the Devonian Woodford Shale and is applicable to other resource shales, in this case to the Permian Upper Wolfcamp Formation in the Central Basin Platform of the Permian Basin. The stratigraphy of the distribution of the Upper Wolfcamp on top of the Upper/Middle Wolfcamp Unconformity is similar to that of the Woodford, suggesting a similar origin and distribution. The resulting stratigraphy in both cases resembles that of the classical Exxon sea slug model except that rather than a single organic-rich deposit defining the condensed section and maximum flooding surface, a second organic-rich deposit occurs stratigraphically lower, at or near the unconformity surface. This theoretical summary can support the discovery of potential drillable target zones in the Woodford Shale and the Wolfcamp Shale.

Syn-thrusting, near-surface flexural-slipping and stress deflection along folded sedimentary layers of the Sant Corneli-Bóixols anticline (Pyrenees, Spain)

In the Spanish Pyrenees, the Sant Corneli-Bóixols thrust-related anticline displays an outstandingly preserved growth strata sequence. These strata lie on top of a major unconformity exposed at the anticline's forelimb that divides and decouples a lower pre-folding unit from an upper syn-folding one. The former consists of steeply dipping to overturned strata with widespread bedding-parallel slip indicative of folding by flexural slip, whereas the syn-folding strata above define a 200 m amplitude fold. In the inner and outer sectors of the forelimb, both pre- and syn-folding strata are near vertical to overturned and the unconformity angle ranges from 10 to 30°. In the central portion of the forelimb, syn-folding layers are gently dipping, whereas the angular unconformity is about 90° and the unconformity surface displays strong S–C shear structures, which provide a top-to-foreland slip sense. This sheared unconformity is offset by steeply dipping faults, which are at low angles to the underlying layers of the pre-folding unit. Strong shearing along the unconformity surface also occurred in the inner sector of the forelimb, with S–C structures providing an opposite, top-to-hinterland slip sense. Cross-cutting relationships and slip senses along the pre-folding bedding surfaces and the unconformity indicate that regardless of its orientation, layering in the pre- and syn-folding sequences of the Sant Corneli-Bóixols anticline were continuously slipped. This slipping promoted an intense stress deflection, with the maximum component of the stress tensor remaining at low angles to bedding during most of the folding process.

Meticulous Depiction and Genetic Mechanism of Unconformity Belt Structure

Current research on unconformity belt mainly concentrates on the analysis and description of unconformity surface and contact relationship with overlying and underlying strata. Too much emphasis on the division of the “three-layer structure” of unconformity yields obvious disadvantages, i.e., that studies are not only insufficient in the authenticity of architecture but also in the characterization of the combination of the two- and three-dimensional perspectives. Thus, the analysis of genetic mechanism and meticulous depiction is ignored. Accordingly, by making full use of the outcrops and combining previous research findings, this study aims to provide a detailed description and analysis for the unconformity belt structure, and points out the non-uniqueness of the three-layer structure of the unconformity belt. Based on the theory of depositional process and according to the method and principle of the “point-line-surface”, a comprehensive characterization method combined with a kind of “two-dimensional structure” and “three-dimensional structure” is proposed. A two-dimensional structure is a kind of surface structure, while the three-dimensional structure is a kind of body structure, Moreover, this study discusses the recognition significance of the visual unconformity line and visual unconformity surface with respect to the unconformity belt structure. According to the spatial allocation and geological significance of the unconformity belt, two types of models are established in order to depict the structure of unconformity belt better. Combined with the structural elements of the unconformity belt and complied from the viewpoint of the sedimentary environment, the formation mechanism of two-and three-dimensional structures is summarized.

Seismic analysis of paleotopography and stratigraphic controls on total organic carbon: Rich sweet spot distribution in the Woodford Shale, Oklahoma, USA

The Devonian Woodford Shale is a prolific unconventional resource shale for oil and gas. Like many such shales, the Woodford sits atop an unconformity on the surface of underlying carbonate rocks (mainly the Hunton Group in this case). There is variable topographic relief on the unconformity surface due to incised valleys, cave collapse, and/or karst formation during periods of subaerial exposure resulting from eustatic sea-level fluctuations. Anomalously high thicknesses of the Woodford, with relatively high total organic carbon (TOC), can form within topographic depressions on the unconformity surface, giving rise to potential “sweet spots” as drilling targets. It is likely that the topographic relief that formed during subaerial exposure created areas of restricted marine circulation (or possibly hypersaline lakes) during an early fall in the sea level, and thus, localized anoxic depositional environments conducive to preservation of organic matter (TOC). Seismic analysis, calibrated with well logs and cuttings, of two areas on the Cherokee Platform in Oklahoma were completed to test the discontinuous and isolated distribution, vertically and horizontally, of the TOC. In one area, the TOC ranged up to 10 wt.% and in the other area, up to 13 wt.%. Seismic inversion and attribute analysis demonstrated the patchy distribution of the TOC vertically and laterally in both areas. These patchy, discontinuous distribution spotlights areas where TOC was preserved (in the minibasins), and point to potential sweet-spot locations. The delineation of organic-rich sweet spots was accomplished by integrating geologic, geochemical, and geophysical data in probabilistic neural networks obtaining seismic impedance-derived TOC that was mapped across different locations in the Cherokee Platform.