Dipmeter Data Research Articles

The present-day stress field and paleo tectonics in offshore Brunei Darussalam

Present-day maximum horizontal stress orientation is one of the fundamental inputs for well design and stability analysis. Since the previous stress orientation map in Brunei Darussalam published in 2005, more borehole image data have been acquired to evaluate both reservoir geology and hole damages of different causes. The image-based breakout orientations in recent wells not only filled critical data gaps but also replaced the low-confident stress orientations from dipmeter data on the previous map. Integration with seismic interpretation also gives new insights into the present-day stress variations in relations to the tectonic evolution.The new map suggests three main stress domains transitioning to each other. Domain A in the deepwater to shelf slope region is compressional, characterized by northwest-southeast maximum horizontal stress, consistently perpendicular to the associated delta toe thrust structures near the well locations. The shelf to onshore region is divided into domain B in the east and domain C in the west as result of different evolutions and interactions between shallow deltaic and deep plate tectonics. This contrasts with the 2005 stress map, which divided the region into inversed inner shelf and extensional outer shelf provinces. The domain B is possibly in strike slip state with margin normal maximum horizontal stress, which is a reversal from the paleo extensions expressed by collapse faults and a large counter regional normal fault. The counter regional normal fault is interpreted as the result of rapid sediment load over thick ductile shales prior to the toe thrust system. The domain C in the west exhibits extension with east-west maximum horizontal stress, parallel to the normal faults but oblique to the thrust faults and the coastline. It is reversed from the latest compressional episode. Restoration of the structural deformation history suggests that tectonic reversals occurred several times in the domain C. Between the two shelf domains, where two sets of structural lineaments intersect, the maximum horizontal stress rotates across the two systems.

Integrating facies analysis with dipmeter data to characterize point bars of the Lower Cretaceous McMurray Formation, Christina River, AB, Canada

The Lower Cretaceous McMurray Formation contains crude bitumen reserves of the Athabasca Oilsands that formed by channel amalgamation and lateral migration of fluvio-tidal point bars. These point bars are widely accepted as incised valley fills during episodic base-level rises in tidally influenced estuary (Mathison, 2004; Hein et al., 2013). Historically, well logs such as gamma-ray and spontaneous potential were used in combination with core data to interpret facies and to conduct stratigraphic correlations of the McMurray Formation. Microresistivity image logs, dipmeter logs, and seismic data have greatly improved the identification of facies and recognition of point-bar geometry. In comparison to microresistivity image logs and seismic data, dipmeter data is more regionally accessible; the tool indicates the orientations of fractures, bedding planes and contacts and it is commonly used to delineate boundaries between different point bars in the McMurray Formation. This work provides examples of dipmeter tadpole analyses and discusses how dipmeter-log interpretations can be supported with core-based facies analyses.This study integrates dipmeter tadpole patterns, sedimentological and ichnological characteristics from 47 sedimentary cores. Vintage dipmeter tadpole patterns were observed and digitized from 52 wells to establish a detailed facies characterization with a particular emphasis on McMurray point-bar complexes and sheltered shoreface/delta deposits. In particular, dipmeter data are useful in indicating paleocurrent directions and lateral accretion orientation of point bars, which helps to reconstruct paleodepositional history and stratigraphic relationships. Point bars in the study area are characterized by these sedimentological and dipmeter characteristics: 1) a lower point-bar section characterized by trough cross-bedded sandstone (locally inclined stratified master bedding) with local mud clasts and disorganized, low-to high-angle dipmeter readings; 2) a middle section represented by inclined heterolithic stratified lateral accretionary deposits that exhibit consistent low-angle dips (5°–18°), and display subtle shallow-to-steep-to-shallow dipmeter pattern; and 3) an upper section of the point bar or in some cases abandoned channel strata comprising laminated and lenticular mudstone or bioturbated heterolithic intertidal strata, each showing low-angle (<4°) dipmeter measurements.The dipmeter characteristics of estuarine point bars are integrated with core observations to delineate distinct point-bar stacking patterns and stratigraphic units. In particular, dipmeter tadpole patterns help to refine facies interpretations, such as cross-stratified sandstone facies. Similar point bar scales and stacking patterns are observed in four channel sets. Point bars show either NW-SW or NE-SE lateral accretion surfaces, suggesting northward paleocurrent flow direction.The analyzed datasets support a transgressive fill incised valley fill model that is consistent with earlier McMurray studies in different areas. The four channel sets descend from different stratigraphic levels, but share similarities in scale, orientation and sedimentology/ichnology. The similarity attests to persistent transgression over McMurray time punctuated with regular sea-level fall and rise until the McMurray Formation strata were replaced by Clearwater Formation sedimentation.

Siliciclastic depositional model within a subsurface coastal parasequence based on barriers and baffles – An application to the Donghe Sandstone Member in the Hudson Oilfield, Tarim Basin, NW China

Detailed analyses of a siliciclastic coastal reservoir highlights the need for an enhanced knowledge of their depositional model. At the scale of the bed sets, however, it is difficult to determine the depositional model, largely due to a limit in vertical resolution of seismic data and conventional wireline logs. Barriers and baffles, following either surfaces of bed sets or beds, act as isochronous boundaries, therefore, holding a key to reveal the sedimentary evolution of subsurface reservoirs at small scales. Calcareous and argillaceous barriers and baffles of the Donghe Sandstone Member in the Hudson Oilfield have obvious low conductivity responses in dipmeter data, therefore, they are interpreted and correlated by integration of conventional logs and dipmeter data. Barriers separate the Donghe Sandstone Member into three retrogradational bed sets (i.e., B1~B3) and two progradational bed sets (i.e., B4~B5), of which beds are constrained by baffles with larger dip angles than barriers in the current structure pattern as well as the period of deposition. The stacking patterns indicate relative sea-level rise within B1~B3 and fall within B4~B5. The relative sea-level changes control the baffle frequency, thickness and length of barriers and baffles. Vertically, the variation of frequency and maximum thickness of both argillaceous and calcareous baffles increase during the transgression yet reduce during regression. Horizontally, the length of barriers and baffles positively correlates with their maximum thickness along the paleoshoreline, as it does the direction perpendicular to the paleoshoreline. Therefore, a depositional model of early retrogradation followed by progradation is built for the Donghe Sandstone Member, a siliciclastic coastal parasequence with upper-shoreface and lower-shoreface facies, based on barriers and baffles. This depositional model is embodied with certain principles of reservoir architecture and behavior.

LATE CRETACEOFS SEDIMENTARY ROCK IN BARITO BASIN, INDONESIA: LITHOLOGY, PALEONTOLOGY, AND PALEOENVIRONMENT

The Barito Basin so far known as back-arc basin that formed by the rifting in Early Tertiary, which the oldest sedimentary rock in this basin is believed has a Middle Eocene to Early Oligocene age. However, this research will present new evidence regarding the existence of sedimentary rocks that are older than Cenozoic age in the Barito Basin. This research was carried out on Bongkang-2 well, as the main data, and other five wells which have an indication of the discovery of Pre-Tertiary sedimentary rocks, which are generally located in the northern part of the Barito Basin. Integration of mud log data, petrography, paleontology, and dip-meter data, resulting the identification of lithology, age and depositional environment, and then interpretation of the paleoenvironment of the Barito Basin in the Late Cretaceous is carried out. Based on the analysis of data, it is show that Pre-Tertiary sedimentary rocks found in the six wells analyzed has Cenomanian age, which is indicated by the presence of large foraminifera fossils in the form of Sulcoperculina sp. and Orbitolina sp. in Bongkang-2, Hayup-1 and Hayup-3 wells, as well as palynomorph fossils in the form of Cicatrico- sisporites dorogensis, A. tricornitatus, Aquilapollenites sp., Distaverrusporites margaritus and Classopolis cf. classoidesin Bagok-1 and Bagok-2 wells. In addition, based on lithological analysis, in the Bongkang-2, Hayup-1 and Hayup-2 wells lithology develops in the form of limestone, shale and sandstone, while in the Didi-1, Bagok-1 and Bagok-2 wells lithology develops in the form of shale with sandstone and pyroclastic volcaniclastics rock intercalation. Then, based on the integration of lithology and paleontology analysis, it is known that in the Cenomanian age, terrestrial environments developed in the western part of the Barito Basin, while in the eastern part the shallow marine environment developed.

The statistical eigenvector analysis technique (SEAT) for dip data analysis

Abstract Dip data acquired from image logs (i.e. the dip/azimuth of planar features) are extensively used in the oil and gas industry to assist in interpreting subsurface geology. Gradual and/or abrupt variations in dip data with depth may indicate different structural features (e.g. faults, folds, etc.) or changes to sedimentary systems (e.g. unconformity). The statistical eigenvector analysis technique (SEAT) and accompanying computer application are useful in statistically calculating the orientation and geometry of subsurface structures from dip data. Poles to planes on a spherical projection over a given depth interval may reveal cluster or girdle distributions, which can be interactively analyzed using statistical distribution functions on the unit sphere. SEAT provides a mathematical solution for as few as two data points, and has the potential to be applied to dip data at the meter-scale. The orientation and geometry of various subsurface geological features (e.g. folds, fault drag deformation, etc.) can be interactively calculated over a chosen depth interval. SEAT is applied to a dip dataset acquired from a modern, 8-padded electronic borehole image log and a previously studied dipmeter data from the discovery well of the Railroad Gap Field to illustrate its higher accuracy in determining the orientation and geometry of subsurface structures compared to older methods. SEAT allows for a vastly improved interpretation of subsurface geological features when combined with high-resolution borehole image logs and accurate dip data. Improved understanding of subsurface geometries may allow for better business decisions in future exploration and appraisal of existing assets.

The role of gamma-ray logs in deciphering geochemical and geological aspects of the Rudeis Formation, Gulf of Suez, Egypt

Spectral gamma-ray tools present the concentrations of the three radioactive elements responsible for the natural gamma radiation, which are thorium (Th), uranium (U), and potassium (K), separately. This separation can be interpreted geochemically and geologically to identify some formation characteristics, such as lithologies, clay types, fractures, unconformities, and hydrocarbon potentialities. In this study, the geochemical and geological contributions of the natural spectral gamma-ray spectrometry (NGS) log were interpreted to characterize the Rudeis Formation in well Amal-9, Amal field, the southern province of the Gulf of Suez, Egypt. The interpretation was supported by other logging tools, cuttings description, and Rock-Eval pyrolysis. The study revealed that reducing conditions were predominant during the deposition of most of the stratigraphic sequence of the Rudeis Formation. The sediments of the Rudeis Formation were identified into four lithofacies; each one has its own diagnostic spectral gamma radiation. The lithofacies are composed of sandstone (lowest radiation), limestone, shale, and marl (highest radiation). The U content can be considered as the main contributor to the radioactivity of the Rudeis Formation. Furthermore, there is a relationship between the U and the TOC contents, implying that U can be used as a proxy for organic richness shale and as an indicator for good source rock intervals in this formation. As indicated from several crossplots, the clay minerals are mainly represented by montmorillonite with a low percentage of kaolinite and illite. Several unconformities were detected, based on the dipmeter data and observing the abrupt change in both the Th/K ratio and U curves.

Internal deformation and kinematic indicators within a tripartite mass transport deposit, NW Argentina

The role of mass transport deposits (MTDs) in redistributing sediment from the shelf-break to deep water is becoming increasingly apparent and important in the study of basins. While seismic analysis may reveal the general morphology of such deposits, it is unable to provide information on the detailed geometry and kinematics of gravity-driven transport owing to the limits of seismic resolution. Outcrop analysis of ancient MTDs may therefore provide critical observations and data regarding the internal deformation and behavior during slope failure. One such field area where geometry and kinematics are clearly exposed is Cerro Bola in the Paganzo Basin of northwestern Argentina. This 8km strike section exposes a mid to late Carboniferous succession, comprising fluvio-deltaic sediments, turbidites and MTDs. Our work focuses on the main MTD that is up to 180m thick and is characterized by a silty matrix, containing sandstone blocks and siltstone rafts. Although we consider a single slope failure as the most likely scenario, a possible double failure might also explain the occurrence of a folded turbidite marker in the upper zone of the MTD. The MTD is host to a variety of deformational features such as folding, boudinage, shear zones, allochthonous strata, and secondary fabrics among others. These deformational features vary in intensity, scale and style, both vertically and laterally across the deposit. The vertical variation is the most notable, and the entire deposit can be subdivided into lower, middle and upper zones according to variations in texture and structures, including sandstone blocks, sand streaks and blebs in the matrix, folding on a variety of scales, and shear zones. The middle part of the MTD is characterized by the abundance of siltstone rafts. Various models are proposed for the origin of blocks and rafts within the MTD: erosion of underlying strata; fragmentation of the original protolith; or a mixture of both. Significantly, specific strain cells occur around the blocks, and so the kinematics of deformation structures in the matrix of the MTD are very largely governed by their proximity and position relative to blocks, and may not relate to the overall kinematics of the MTD. This casts serious doubt on the ability to interpret overall movement directions from core or dip-meter data in the subsurface.

Building a valid structural model in a triangle zone: An example from the Neuquén fold and thrust belt, Argentina

Structural modeling and systematic analysis are necessary in complex areas even before seismic interpretation. Seismic data in these zones usually show poor-quality images of the geologic elements. Therefore, a detailed analysis of the available information (e.g., seismic, previous wells, and surface geology) is needed to build a comprehensive structural model to constrain the seismic interpretation. Although a restorable structural cross section is not necessarily the unique solution, it is a valid interpretation to begin with, which can be tested against additional information. As an example, a structural profile interpretation in the Agrio fold and thrust belt of the Neuquén Basin, northeastern Argentina, is presented and described as invalid on the basis of several unrestorable geometric components. This led to the reevaluation of the structure to understand its possible exploratory potential. Integration of different data such as regional structural styles inferred fault trajectories from seismic reflection termination patterns, synorogenic deposit geometries in seismic, and surface geology, dipmeter data to identify the presence and location of fault position and vergence of the fold axis, and a kinematic forward model, resulted in a new restorable structural model. The new interpretation proposes a “broken triangle zone” framed by two opposite surface anticlines with the same detachment level, later faulted by a basement fault. The interpreted model suggests at least two phases of deformation (1) a thin-skinned phase with the two opposite anticlines sharing the same detachment level that is responsible for about 80% of the shortening and (2) a thick-skinned phase from an east-verging basement fault, transporting the western anticline and crosscutting the eastern anticline, that is responsible for 20% of the shortening and the synorogenic deposits at the frontal region. This new interpretation defined a new exploration prospect drilled by a wildcat well, which validated the interpretation and model predictions.

Hydrocarbon potential, structural setting and depositional environments of Hammam Faraun Member of the Belayim Formation, Southern Gulf of Suez, Egypt

The Middle Miocene Belayim Formation is one of the most prolific formations in the Southern Gulf of Suez. It consists of four members; two members are evaporitic (Baba and Feiran) and the other two members are prospective, mostly clastics (Hammam Faraun and Sidri). The hydrocarbon potential and depositional environment of Hammam Faraun Member, the target of the present study, have been studied in 11 wells distributed in the southern province of the Gulf of Suez. The traditional well log data, as well as the Spectral Gamma-Ray logs ‘SGR’ and dipmeter data were used to evaluate the petrophysical properties and distribution of the Hammam Faraun Member in the Southern Gulf of Suez. It varies greatly in thickness with the greatest thicknesses in GS 365 (372 ft) and GS 373 (430 ft) fields in the central parts and the thinnest at the basin margins of the studied area at GH376 (65 ft) and Ras El Bahar (67.5 ft) fields. It is composed of clastic rocks, mainly shales and sometimes reef carbonates. The very good petrophysical properties of the studied sequence indicate a good reservoir in some fields with good to very good porosity (13.5 ≤ ∅ ≤ 25.0%). The shale volume of this reservoir sequence is less than 33% and the water saturation is less than 42.3%, while the net-pay thickness is up to 58 ft.The SGR and Pe logs indicate that, the studied rocks were deposited mostly in lagoonal to shallow marine environments, with illite and montmorillonite as dominant clay minerals.The dipmeter data obtained in some wells indicate slightly tilted beds, mostly less than 20° with an overall dip direction towards the SW, which represents the regional dip in the Southern Gulf of Suez. Based on dipmeter data, two major angular unconformities can be detected; one at the top of the sequence, separating it from the overlying South Gharib evaporates, and another one at the base of the sequence, separating it from the underlying Feiran Member.

STATISTICAL ANALYSIS OF DIPMETER LOGS FROM EXPLORATION WELLS IN THE DRAVA DEPRESSION, NORTHERN CROATIA

Stratal dip and azimuth data from dipmeter logs recorded at wells in the western Drava Depression (northern Croatia) were analysed in order to assist with subsurface structural interpretations. The dipmeter data came from the Miocene Moslavacka gora, Ivanic‐Grad and Kloštar Ivanic Formations at wells Virovitica‐3, Rezovacke Krcevine‐1 istok, Rezovacke Krcevine‐2 and Orešac‐1, located near the town of Virovitica. In contrast to conventional dipmeter interpretations in which dip patterns are represented by “colour trends” (e.g. red represents increasing dip with depth), statistical methods of analysis were used in this study, and consisted of the interpretation of regression curves of dip and azimuth variations with depth. Interpretation of the dipmeter data in this way assisted with the structural interpretation of this folded and faulted part of the Drava Depression.Also discussed are techniques for the selection of typical regression curves which can help in the identification of lithologically similar intervals based on stratal dip and azimuth data. A total of six such curves can be used for structural and depositional interpretation of such intervals.

The improvements of three-dimensional seismic interpretation in comparison with the two-dimensional seismic interpretation in Al-Amal oil field, Gulf of Suez, Egypt

Thirteen 2D seismic lines were interpreted with the help of well velocity and time-depth trace conversion to construct the structure-tectonic maps. This is to characterize the different stratigraphic tops of Al-Amal area, as well as to confirm the validity of the proposed structural model. Most of the available seismic data in Al-Amal area were investigated and reviewed to select the best quality set.In order to study the detailed structural elements based on the 3D seismic lines; six depth structure contour maps were constructed on the tops of Zeit, South Gharib, Belayim, Kareem, Nukhul and Matulla formations from top downward. Interpretation was aided by the missing sections detected from the available well tops and dip-meter data as well. These maps indicate that, both of Miocene and Pre-Miocene formations in Al-Amal field were affected by elongated tilted graben blocks trending in the NW–SE directions and bounded by two sets of faults, which are down throwing toward the west and eastern directions.

Structural evaluation of column-height controls at a toe-thrust discovery, deep-water Niger Delta

Industry discoveries to date in the deep-water toe-thrust play have limited column heights and lack any unequivocal thrust-fault-dependent hydrocarbon columns. The causes of the limited success are controversial, the main issue being whether the encountered columns are caused by leaking thrust faults or other causes. The controversy is in large part caused by the presence of no-seismic-image zones that, even on three-dimensional (3-D) prestack-migrated data, obscure the possible thrust cutoffs. We use dipmeter, seismic, stratigraphic, and fluid pressure data to construct an integrated geometrically and kinematically balanced cross section through a recent toe-thrust discovery in which a prominent no-seismic-image zone on the southwest flank of the Alpha structure appeared to be a thrust fault zone holding about 100 m (328 ft) of thrust-fault-dependent oil column. To constrain the structure within the unimaged forelimb, dip panels and fold axial surfaces were constructed from dipmeter data recorded in both a vertical hole and a sidetrack well through the no-seismic-image zone. Stratigraphic tops from both wells were projected through the no-seismic-image zone using the dip panels and axial surfaces, maintaining the observed stratigraphic thicknesses. These data and analyses tightly constrain potential fault locations and offsets within the no-data zone. The resulting structural model shows that the no-seismic-image zone in the forelimb is not a thrust but instead an overturned limb without any fault offset of the pay section. The results show that the trapped hydrocarbons are confined completely within the four-way dip closure, and that the trap's potential is not limited by the thrust faults' seal capacities. In addition, top-seal analyses show that top-seal integrity is not a limiting factor for hydrocarbon column heights at the present time and has not been a limiting factor throughout the geological history of the structure. Taken together, these observations indicate that the column heights are most likely controlled by access to charge instead of trap integrity, consistent with an observed lack of thermogenic hydrocarbons in nearby four-way dip closures. These conclusions imply that where sufficient access to thermogenic charge is present, there may be additional thrust-fault-dependent hydrocarbon columns.

TrackDip: A multi-scale processing of dipmeter data – Method, tests, and field example for 3-D description of gravity-driven deformations in the Eocene foreland basin of Ainsa, Spain

This paper presents TrackDip, a new method for dipmeter data processing. This method selects, localizes and measures significant tilts from the comparison at various scales of the changes of orientation of sedimentary beddings. We tested this method on simple cases (unconformity, gradual tilt, successive tilts, without and with additional random noise) and on a field example, along three sections in the Eocene Sobrarbe delta, Ainsa foreland basin, northern Spain. For synthetic data sets, the method clearly identifies and measures the introduced tilts. On the field, sedimentary and tectonic structures, especially three main syn-sedimentary sliding surfaces (S1–S3), were successfully identified from processed dipmeter data. The tilt axis are mainly trending N–S in sandstones, associated to Westward transport of sediments. The sliding surfaces S1 and S3 correspond to E–W-trending tilt axis, tentatively correlated either to the flexural subsidence of the basin, or to anticline growth during sedimentation South of the studied area. Combination of these sedimentary and tectonic directions results in a NE–SW striking submarine slope, which locally controls the gravity-driven deformations, especially on the S2 sliding surface. Finally, NNW–SSE-trending tilts above the uppermost S3 sliding surface are interpreted as the result of infilling on the side of the scar produced by sliding.

Three-dimensional structural model of the Painter and East Painter reservoir structures, Wyoming fold and thrust belt

The Painter and East Painter reservoir structures are located in the hanging wall of the Absaroka thrust in the Wyoming fold-thrust belt. Balanced cross sections and interpreted logs and dipmeter data from more than 50 wells have been integrated to develop a three-dimensional (3-D) structural model of the Painter reservoir structures. The structures are interpreted as a pair of asymmetric faulted detachment folds formed along the hanging-wall ramp in Triassic–Jurassic units in the Absaroka thrust sheet. The Painter reservoir structure verges to the southeast and has a gentle backlimb and a steeply dipping to overturned forelimb, whereas the East Painter structure displays steep dips on both limbs. The front limbs of both structures contain forelimb thrust faults with small displacements. A tight syncline separates the structures and contains several out-of-syncline thrusts in the Jurassic Twin Creek limestones. Cross sections through the structures are restored using line-length balancing for the Nugget formation and area balancing of the Ankareh-Thaynes-Woodside formations and the Twin Creek Formation. The structures are interpreted to have initiated as a pair of detachment folds cored by moderately ductile limestones and shales of Triassic age. Increasing shortening resulted in tightening of the structures, the development of out-of-syncline thrusts, and the propagation of thrust faults on the steep forelimbs. The tight geometry of the East Painter structure resulted from frictional resistance to fault slip along the hanging-wall ramp in the Triassic–Jurassic units. The balanced 3-D structural model ensures consistency of the geometry of all interpreted horizons and faults and can be used to construct improved structural maps of reservoir units in the Painter reservoir structures.

南長岡ガス田周辺の火山岩貯留岩の分布と特性

The distribution pattern of the Green Tuff rock type and facies in the Niigata Sedimentary Basin, with a central focus on the ‘Nagaoka Area’, may have been controlled by rifting and other structural development in the Nanatani Stage. The term “Volcanic Sequence” model is now propounded as a volcanostratigraphic framework for the compound Green Tuff succession which encountered in the Minami Nagaoka Gas Field, based on the comprehensive interpretation and crossed understanding of a large variety of volcanic rock types and facies and their changes in physicality and geochemical composition. The model comprises five volcanic sequences such as the Volcanic Sequence I, II, IIIR, IIIB and IV in ascending order. The Volcanic Sequence I and II consist of non-acidic rocks, and the former characterized with an extremely high resistivity is sharply distinguished from the latter. The acidic Sequence IIIR which has a bimodal relationship with the basaltic Sequence IIIB consists of multi-stacked rhyolitic lava-flows and/or lava-domes. Those are able to be identified based on the composite interpretation of the presumed rock facies together with dipmeter data in each well. The Sequence IV, a thin basaltic flow unit, is limited its distribution to the northern part of the field. Reservoir geological interpretation based on the “Volcanic Sequence” model might be able to explain heterogeneous facies distribution, and different productivities and pressure performances among wells..In the ‘Nagaoka Area’, some of the vintage wells encountered acidic rocks corresponding to the Volcanic Sequence IIIR and the lowermost part of the underlying non-acidic rocks are extremely resistive like the Volcanic Sequence I. In order to predict the distribution of acidic volcanic bodies which remain undiscovered, it is worth trying to re-interpret the vintage wells and seismic data from the aspect of the “Volcanic Sequence”

The Corrib gas field, offshore west of Ireland

The Corrib Gas Field in the Slyne Basin lies about 70 km west of Co. Mayo, offshore Ireland. The basin is a narrow Triassic/Jurassic half-graben and is one of a series of structurally linked basins that are present along the west coast of Ireland and the British Isles. The field was discovered in 1996 by well 18/20-1, encountering a 61 m gas column in low-porosity Triassic fluvial sandstones at a subsea depth of 3539.6 m. Using extremely poor 2D seismic data, it was originally mapped as a tilted fault block, with an associated rollover structure to the east. The data quality was particularly poor due to the near-surface Palaeogene volcanics that contribute to energy dispersion and extreme multiple generation. A 3D seismic survey was acquired in 1997 and resulted in a major improvement in data quality, allowing improved structural interpretation that showed the Corrib Field to be a faulted anticline. The subsequent appraisal well, 18/20-2z, drilled approximately 1 km from the subsurface location of 18/20-1, penetrated a 185 m gas column in fair to good quality sandstone. It tested dry gas at a stabilized rate of 62.87 × 10 6 SCF/d on a 2″choke. Reprocessing of the seismic data, along with the successful drilling of the following well 18/25-1, confirmed the field to be a relatively simple anticlinal trap, with a complex faulted overburden that is structurally detached from the reservoir by the Mercia Halite. The source rock is assumed to be the Westphalian Coal Measures, similar to that encountered by well 27/5-1. The dry gas is consistent with a Type-III humic source rock. The reservoir is biostratigraphically barren, consisting of fluvial red bed sandstone, and is assumed to be of Triassic Sherwood Sandstone Group equivalent age. The unit is approximately 400 m thick, consisting of a high net-to-gross sequence of low-sinuosity braided fluvial channel sandstones with subordinate sand-flat and playa mudstone deposits. On a macro scale, it is a remarkably uniform sandstone with only subtle facies variations, but on a micro scale there is a significant variation in grainsize and cementation, which affects reservoir productivity. Mineralogically, there are distinct differences from the Sherwood Sandstone Group of the East Irish Sea, indicating that the Slyne Early Triassic system was depositionally similar to, but distinct and separate from, that to the east of Ireland. Dipmeter data indicated that the river system flowed from the SW to NE, essentially along the direction of the present-day Slyne Basin, suggesting that it was largely sourced from the south in the Variscan hinterland, but with evidence of sediment input from local highs such as the Connemara Massif.

From seismic data to core data: an integrated approach to enhance reservoir characterization

Abstract Integrated structural analyses of seismic and various well data are necessary to optimize hydrocarbon reservoir characterization. However, there are many published examples from the oil and gas industry where single data types are analysed but not integrated. This may lead to erroneous interpretations and drainage strategies. As illustrated by an example from the area around well 34/10-B-12 in the North Sea Gullfaks Field, integrated structural interpretation should typically utilize all available seismic surveys, well log correlation data, dipmeter data and core data. Interpretation of seismic data helps in the understanding of large-scale structural and stratigraphic geometries. Time-lapse (4D) seismic helps to identify changes in reservoir properties caused by injection and production. Well log correlation data are used to document variations in zonation thickness caused by sedimentological or structural changes. Dipmeter data tie observations of bedding orientation from seismic data to subseismic scale. Core data represent the most detailed (millimetre to metre scale) data available and can yield information on rock properties as well as sedimentological and (micro)structural features. Small-scale deformation structures such as deformation bands and fractures can typically be identified and characterized. In addition, it is possible from unorientated cores to find the orientation of bedding and deformation structures. This information is compared to observations from dipmeter data, well log correlation data and seismic data to improve the interpretation. Well 34/10-B-12 is a hanging wall injector near one of the large-scale faults in the Gullfaks Field. Several 3D seismic surveys are available from the area, as are standard well log data, dipmeter information and cores. Together, the data range from millimetre to kilometre with some overlap between the data types. Through integrated analysis, pitfalls such as interpreting any linear feature on timedip attribute maps as faults has been avoided. Also, a geometric relation between core-scale and seismic-scale faults has been established, and it has been possible to relate small-scale and large-scale structures in a model which is consistent with all the available data.

Along‐depth stress rotations and active faults: An example in a 5‐km deep well of southern Italy

We have analyzed a 1500 m section at 3.9 to 5.4 km depth in a well of the southern Apennines, in order to better characterize the local active stress field and its correlation with tectonic structures. In this paper we present and discuss the results obtained from the comparison between breakouts and structural analysis from dipmeter data. We have found that the mean breakout direction is in agreement with the regional stress field that in this area is characterized by normal faulting (σ1 = σv) with NE‐SW trending extension (horizontal σ3). Since the regional stress field is relatively well known in this region, we could detect and study some anomalous horizontal stress directions along the well, which we interpret as due to faults crosscutting the borehole. A detailed comparison between the breakout‐inferred stress variations along depth and the faults identified by the dipmeter analysis reveals that some of these faults are associated with stress rotations, whereas others do not show any variation. The former can be interpreted either as “open” fractures or as faults that slipped recently with a near‐complete stress drop, and the latter can be interpreted as “sealed” faults. In particular, we found that the main thrust faults of the area, mainly active in Pliocene times, appear to be sealed, whereas ∼E‐W trending high‐angle (normal?) faults determine strong stress rotations, suggesting that they are the main active structures of the region. This suggests that the study area is located in a transfer zone between the two main “Apenninic” (NW‐SE trending) fault systems which ruptured in the last 150 years. This study has shown that a detailed analysis of the structural and geometrical characteristics of deep wells can be used for the reconnaissance of active structures. This approach can contribute to seismic hazard studies and, if carried out in an oil‐bearing section, can help to maximize the hydrocarbon production.

Structural core analysis from the Gullfaks area, northern North Sea

Comprehensive analyses of more than 8 km of core data from the Gullfaks area, northern North Sea, have proven invaluable for a thorough understanding of the detailed deformation characteristics. In addition, the integration of the results with analyses of dipmeter data, well log correlation data, seismic data and field analogue data yields important information on the general structural characteristics and reservoir properties. Microanalyses show that all faults in the reservoirs are associated with abundant shear bands (deformation bands) clustered in a narrow damage zone that seldom exceeds a few tens of metres in width. Where mica is abundant, the bands are dominated by phyllosilicate framework structures, whereas disaggregation structures dominate cleaner sandstones. Permeability reduction across the shear bands depends on the amount of phyllosilicates present and is generally negligible in clean sandstones. Only minor amounts of cataclasis are observed. The width of the shear bands increases with increasing grain size. Orientation analyses of the dip of shear bands associated with minor faults show that these are symmetrical around a vertical axis. Since the general bedding rotation as observed in well data and seismic data is 10° towards the west, most shear bands must have formed after the rotation of bedding. This is likely because minor faults formed as (late) accommodation structures during slip along the main faults. The reservoir rocks are affected ductilely (by fault drag) in a zone wider than that affected by shear bands. This shows that part of the deformation was by a homogeneous redistribution of individual grains rather than by shear bands or discrete faulting. The integrated analyses of core data with seismic data demonstrate that most (curvi-)linear and fault-like features observed on seismic attribute maps are in fact noise-related.

Abstract: Analyzing structural play leads using dipmeter data

Abstract: Analyzing structural play leads using dipmeter data