Structural features in the Miocene sediments of the Pécs-Danitzpuszta sand pit (SW Hungary)

The Pécs-Danitzpuszta sand pit in southern Hungary exposes middle and upper Miocene (Badenian to Pannonian/Langhian to Tortonian) sediments along the mountain front fault zone of the Mecsek Mts and preserves an essential record of tectonic events during and after the early late Miocene, which are not exposed elsewhere in the region. In this paper we present structural observations recorded over 20 years of work, date the deformation events with mollusk biostratigraphy and make inferences on the structural evolution of the area. At the beginning of the time interval between 10.2–10.0 Ma, NNW–SSE (to NW–SE) extension created normal faults and negative flower structures. These show that extension-related fault activity lasted here up to the late Miocene. Shortly thereafter, still in the early part of the time interval between 10.2–10.0 Ma, N–S to NNW–SSE compression ensued and dominated the area ever since. Deformations under this stress field included reverse faulting in the Pannonian marls and sands, folding of the whole succession, with bedding-plane slip and shearingelated block rotation in the already deposited middle and upper Miocene marl layers and continuously changing bedding dips and southward thickening layers in the Pannonian sands. Lake level changes of Lake Pannon must have played a role in the formation of an angular unconformity within the sands besides compression. The compressional event can be explained by the Africa (Adria) – Europe convergence, but cannot be correlated regionally; it pre-dates basin inversion-related events reported from the region so far.

An innovative method to simulate stress-induced velocity changes in anisotropic rock

This contribution proposes a numerical microstructural modeling approach to investigate stress-induced seismic velocity changes on anisotropic rocks. By introducing pre-existing cracks with preferential orientations in bonded-particle assemblies, the transverse isotropic structure of the Whitby Mudstone is simulated. Using power-law distributed aperture and calibrated micro-properties, we successfully reproduce stress-dependent velocity changes on Whitby Mudstones with different anisotropic angles in relation to the applied loads. The proposed model also duplicates the directional dependence of wave speed with respect to the bedding plane as expected theoretically. The numerical models show that velocity increase results from the closure of pre-existing cracks due to load increase. Direct relations are established between velocity changes and opened crack density (or crack closure), which displays a similar tendency compared with theoretical predictions. This relation can be used to quantify the micromechanisms behind the velocity changes. The proposed model provides the ability to directly examine the micro-processes underlying velocity changes.

The Investigation on Initiation and Propagation of Hydraulic Fractures in Shale Reservoir

Hydraulic fracturing is a necessary technique for shale gas exploitation. In order to have efficient stimulation treatment, a complex fracture network has to be developed, whereas with rich bedding planes and natural fractures, the mechanism of forming a fracture network is not fully understood and it is so tricky to predict propagation and initiation of hydraulic fracture. Therefore, in this paper, considering the strong anisotropy of shale reservoir, numerical simulation has been conducted to analyze fracture propagation and initiation on the basis of finite element and damage mechanics. Simulation results indicate that hydraulic fracture is not merely controlled by in situ stress due to strong anisotropy in shale. With plenty of bedding planes, hydraulic fracture tends to have initiation and propagation along the bedding plane. In particular, this influence becomes stronger with low strength and high development density of bedding planes. Additionally, in combination with natural fracture and bedding plane, the initiation point is usually on a natural fracture plane, causing relatively small breakdown pressure. In the process of fracture propagation, hydraulic fracture connects with natural fractures and bedding planes, forming dendritic bifurcation and more complicated paths. Numerical simulation proves that bedding plane and natural fracture are vital factors of hydraulic fracture. Compared to natural fracture, the bedding plane has a stronger impact on hydraulic fracture propagation. For the initiation of hydraulic fracture, natural fracture is the major effecting factor. The outcome of this study is able to offer theoretical guidance for hydraulic fracturing in shale.

Ichnofabric analysis as a tool for characterization and differentiation between calcareous contourites and calciturbidites

ABSTRACT The Eocene–Miocene Cyprus paleoslope system records complex deep-marine sedimentation comprising background vertical settling of autochthonous pelagic–hemipelagic particles (chalks) which were punctuated by calcareous bottom currents (contourites) and gravity flows (calciturbidites). The Eocene Lefkara Formation at the Petra Tou Romiou beach section (Cyprus) shows the incidence of deep-marine bottom currents and distal turbiditic episodes in a context of pelagic–hemipelagic sedimentation. Trace-fossil analysis of this section, using an ichnofabric approach (i.e., ichnodiversity, Bioturbation Index, Bedding Plane Horizontal Index and crosscutting relationships), was conducted to precisely describe the paleoenvironmental conditions of this complex setting. Ichnofabric analysis allow the characterization and differentiation of sporadic turbiditic events that disrupted both pelagic–hemipelagic and contourite deposition. Calciturbidite intervals show ichnofabrics consisting of postdepositional U-shaped traces (i.e., Arenicolites isp., ?Diplocraterion isp.,) and vertical borings typical of consolidated substrates. High-energy sandy contourite deposits are dominated by horizontal deposit-feeder traces and the development of ichnofabrics with Planolites isp., and Thalassinoides isp. The record of ichnofabrics with slightly deformed Planolites in the interbeds of sandy contourites or in the transition between the facies reveals variations in sedimentation in the bi-gradational contourite succession, and can potentially act as an indicator of depositional hiatus.

Elastic anisotropy of shales: the role of crack alignment and compliance ratio

Cracks, broadly defined as compliant discontinuities, are a major cause of elastic anisotropy. However, few models are available for quantifying crack properties relevant to anisotropy. We developed a rock physics model to quantify crack angular distribution and normal-to-tangential compliance ratio from pressure-dependent acoustic velocities measured in the laboratory. The proposed model utilizes a rectangular function of variable width and amplitude to extract the maximum dip angle of cracks, which is a direct quantification of crack alignment relative to the bedding plane. We tested the model on an organic-rich shale dataset and confirm that both crack alignment and compliance ratio strongly impact Thomsen anisotropy parameters, thus demonstrating the model as a useful tool for better understanding how cracks affect elastic anisotropy.

Experimental Investigation on the Initiation of Hydraulic Fractures from a Simulated Wellbore in Laminated Shale

Understanding the formation mechanisms of complex fracture networks is vitally important for hydraulic fracturing operations in shale formation. For this purpose, a hydraulic fracturing experiment under a core-plunger scale is conducted to investigate the impact of the bedding plane angle, borehole size, and injection rate on fracture initiation behaviors of laminated shale rock. The results on rock properties demonstrate that the anisotropic characteristics of shale rock are reflected not only in elastic modulus but also in tensile strength. The results of fracturing experiments show that the bedding plane dip angle and borehole size have significant effects on fracture initiation behaviors, in that fracture initiation pressure (FIP) decreases with the increase of those two factors. The impact of injection rate, by contrast, has no obvious variety regulation. The above data is further used to validate our previously proposed fully anisotropic FIP model, which shows better agreement with experimental results than those using other models under various parameter combinations. Finally, a postfracturing analysis is performed to identify the fracture growth patterns and the microstructures on the fracture surfaces. The results show that the hydraulic fractures (HFs) always grow along mechanically favorable directions, and the potential interaction between HFs and bedding planes mainly manifests as fracture arrest. Meanwhile, the roughness of fracture surfaces is physically different from each other, which in turn results in the difficulties of fluid flow and proppant migration. The findings of this study can help for a better understanding of the fracture initiation behavior of laminated shale rock and the corresponding fracture morphology.

New Technology Yields High-Resolution 3D Modeling for Fracture Analyses

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 203108, “High-Resolution Fracture Analyses and 3D DMX DFN Modeling of Triassic Dolomites, Wadi Bih, Ras Al Khaimah, UAE,” by Janpieter van Dijk and Raffik Lazzar, GeoModl, prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually 9–12 November. The paper has not been peer reviewed. The complete paper outlines a high-resolution 3D fracture modeling exercise using the DMX protocol applied to Triassic dolomites of the United Arab Emirates. The outcropping rocks show a low primary porosity, are well bedded, and are highly fractured (jointed) up to centimeter scale. The exercise shows the relevance of applying new technologies to outcrop observations and shows several elements and related technologies that, to the authors’ knowledge, have not been presented previously. Introduction The focus area of the complete paper is a small outcrop situated in Wadi Bih in the territory of Ras Al Khaimah (Fig. 1) along a small road near a recently constructed artificial lake. This outcrop, which is approximately 150 m2 in size, shows well-bedded, highly fractured Triassic dolomites. Both section views and bedding-plane views can be observed. The outcrop was selected because it represents an analog of the Triassic Khuff formation, an important hydrocarbon-producing reservoir in the region. The outcrop is easily accessible and displays a clearly defined fracture (joint) network with recognizable sets, also showing truncation relationships between fractures, joints, and bedding that can be examined. Geological Context The area shows a complete series of Permian to Cretaceous, mostly carbonate sediments, outcropping in a series of north/south to north-northeast/south-southwest anticlines and synclines bounded by mostly west-vergent thrust faults. The Wadi Bih outcrop is situated on the moderately east-dipping flank of the north/south-trending Hagab Anticline, also called the Hagil Window after the area of the nearby Wadi Hagil, where the deepest Permian series are outcropping in the core of the anticline. This anticline is situated on the foot-wall of a major north/south-trending thrust fault. The geological history of the area is connected to the initial Mesozoic deposition of the series on the shelf area along the northeast flank of the Arabian shield. In the outcrop study, the focus is on the joint network. The authors write that this network is tilted together with the bedding as part of the flank of the anticline. No relation can be detected between the joint network sets and the fault and anticline axis pattern dominating the area. The joint network, therefore, most probably was formed in the early stages after lithification and dolomitization of the rock.