Petrophysical Datasets Research Articles

Diagenetic impact on unconventional tight carbonate reservoir of AR-F Member from the Abu Gharadig Field, north Western Desert, Egypt: insights from integrated combined sedimentological, petrographic, and petrophysical dataset

Diagenetic impact on unconventional tight carbonate reservoir of AR-F Member from the Abu Gharadig Field, north Western Desert, Egypt: insights from integrated combined sedimentological, petrographic, and petrophysical dataset

The search for “Jenyon’s Channel”: The missing link between the Permian basins in the North Sea

The Mid North Sea High (MNSH) Seaway, otherwise known as Jenyon's Channel, was the only major marine connection between the Northern and Southern Zechstein Basins during the latest Permian. Current understanding favours a model where marine replenishment began with a northern connection to the Panthalassic Ocean (along a network of basins that were precursors to the North Atlantic Rift System), after which marine water passed into the Northern Zechstein Basin, travelled through the MNSH Seaway before finally reaching the Southern Zechstein Basin. This study delineates evaporite formations in an extensive petrophysical dataset to analyse marine connections and whether they influenced Zechstein facies distribution. The first four Zechstein Cycles (Z1-Z4) are identified on the MNSH platform which shows that this structure was always covered by a thin water column during sea-level highstand; however, during Z2 sea-level lowstand, the MNSH Seaway provided the main connection between the basins. As relative sea-level fell, the MNSH Seaway became increasingly constricted resulting in hypersalinity due to limited volumes of marine water reaching the Southern Zechstein Basin. This system precipitated vast volumes of Z2 (late Wuchiapingian) halite in the Southern Zechstein Basin, whereas this facies remains reduced in the Northern Zechstein Basin. In Z3 and Z4 (Changhsingian) times relative average sea-level was higher resulting in sustained communication between the basins, even in times of sea-level lowstand. This is evidenced through a regionally traceable blanket of Z3 halite, along with occasional examples of Z2 carbonate platforms becoming entombed in Z3 halite. This work provides revised palaeoenvironmental understanding for the latest Permian. The workflow could also aid in the study of other oceanic gateways with further implications for hydrocarbon exploration efforts and emerging energy transition technologies such as subsurface storage on the UK Continental Shelf.

Deep low-salinity groundwater in sedimentary basins: petrophysical methods from a case study in Somalia

Oil and gas exploration data for the Northern Somalia and Horn of Africa region have been utilised for deep freshwater resources exploration. This unique geophysical and petrophysical dataset is integrated into a regional hydrogeological study and it allows for targeting of unmapped and unknown deep aquifers that would otherwise be beyond the conventional reach of the groundwater sector. The database includes open-hole well logging surveys (resistivity, gamma ray, and spontaneous potential), mud logging and drill stem test (DST) data. The results from the data analysis are combined with the geological interpretation to assess the freshwater potential of each mapped hydrostratigraphic unit in the region. The results highlight the presence of several previously unknown deep low-to-medium salinity (<9 g/L total dissolved solids (TDS)) aquifers within the Jurassic to Eocene units, including the Adigrat, Hamanlei, Gabredarre, Gumburo, Jessoma, and Auradu formations. With tested intervals of water containing less than 3.8 g/L total dissolved solids, the Auradu, Jessoma, and Gumburo formations are the most promising deep freshwater aquifers. Additional results on the analysis of groundwater parameters, such as hydraulic head, provide the basis for further groundwater modelling. The results show that the method has good potential, in particular for use in drought-stricken areas of arid regions. Ultimately, the results from this region have global significance, as the method can provide a new means of boosting fresh groundwater resources in water-poor regions, and supporting sustainable development and utilization of the resources in the medium and long term.

Diagenetic overprint on porosity and permeability of a combined conventional-unconventional reservoir: Insights from the Eocene pelagic limestones, Gulf of Suez, Egypt

Lower to Middle Eocene organic-rich deep-water limestones of the ∼335-m-thick (1100 ft) Radwany (Thebes-equivalent) Formation represent a source rock and a potential reservoir unit at the October Oil Field in the Gulf of Suez. However, in spite of recent exploration advances, the pore system and diagenetic history of the formation are still poorly understood. This study aims to discriminate porosity types and their vertical distribution, assess the diagenetic processes controlling porosity evolution, and evaluate the unit's overall reservoir potential. To achieve these goals, we utilized an integrated petrographic, mineralogical, geochemical and petrophysical dataset from four offshore wells. At October Oil Field, the Radwany Formation is informally divided into three intervals (from the base up: C, B and A) which are composed of planktonic foraminiferal wackestone, wackestone-packstone and packstone microfacies. Pores include both fabric selective (interparticle, intraparticle and moldic) and non-fabric selective (fracture) types. Visible porosity reaches up to 11.6%, and the highest porosity values are associated with interparticle pores in foraminiferal packstone in interval C. The two overlying intervals have lower porosities but have TOC values of up to 6 wt %. The Nuclear Magnetic Resonance (NMR) analysis yields low amplitudes of short T2 components (max. 52 ms), which suggests the presence of small pores (intraparticle, moldic, fracture, and fissure). Based on the electrical quality index (EQI) rock classification, nine groups were identified, suggesting a high heterogeneity of the formation. The highest EQI values positively correlate with high porosity (visible and estimated) in the lower part of Interval C which is dominated by interparticle/intraparticle porosity, thus reflecting the low tortuosity values of this microfacies. Dominantly low EQI values in Intervals A and B indicate low porosity efficiency and higher tortuosity. Carbonate sediments of the Radwany Formation are interpreted to have undergone a complex series of diagenetic processes. These processes modified the primary pore system either by enhancing the reservoir properties (dissolution and fracturing), or by reducing or destroying porosity (cementation, mechanical and chemical compaction, and minor pyritization, chertification, and asphaltene precipitation). The study highlights the unconventional reservoir potential of the middle and upper parts of the Radwany Formation (intervals B and A) at October Oil Field, and the conventional reservoir potential of interval C. It emphasizes the complexity of diagenetic controls on the porosity evolution in deep-water limestones of the Gulf of Suez, and may provide valuable insight into similar deep-water carbonate systems in rift basin reservoirs elsewhere.

Permeability prediction in argillaceous sandstone reservoirs using fuzzy logic analysis: A case study of triassic sequences, Southern Hassi R'Mel Gas Field, Algeria

Discriminating the argillaceous sandstone reservoirs into several hydraulic flow units (HFUs) is a useful reservoir zonation technique. This study introduces a statistical method for analyzing petrophysical data sets, including borehole-logs and core data, to discriminate the main Triassic gas-producing argillaceous sandstone reservoirs in Hassi R'Mel Northern Field in Algeria into some HFUs.These Triassic Formations consist mainly of argillaceous sandstone, sandy shales, dolostones, and evaporite intercalations. Integration between the X-Y plot of porosity and permeability data, and their frequency distribution histograms introduced a diagnostic reservoir mathematical model for predicting both parameters. On the other side, the petrophysical model framework that based on log responses indicates the ability to cluster log responses of the Triassic Hassi R'Mel formations into many clusters and components. The reservoir characterization workflow of Hassi R'Mel formations started with processing the log responses of eight logged boreholes, and some high reliable mathematical models (R2 = 0.943) were introduced to estimate permeability in the un-cored intervals. Besides, applying a fuzzy logic technique enabled a reservoir zonation of the Southern Hassi R'Mel Gas Field into several HFUs with various reservoir properties. Predicted permeability values of each flow unit indicate high reliable relationships established between the measured and calculated permeability using the fuzzy logic technique.

Efficient regional scale 3D potential field geophysical modelling to redefine the geometry of granite bodies beneath prospective, geologically complex, northwest Tasmania

In this study, a regional model that defines the three-dimensional geometry of the subsurface geology beneath the complex, prospective northwestern Tasmania has been developed. This has been achieved using a series of potential field inversions constrained by surface geology, geological sections, seismic interpretations and a newly extended petrophysical dataset. Three major episodes of granitic magmatism are preserved in Tasmania: in the Neoproterozoic, Cambrian and Devonian. Granite bodies are hence considered important indicators of mineralization for explorers in an area of challenging vegetation, topography and cover sequences. Forward modelling and property-based inversions of the pre-existing geological model show that the previously interpreted subsurface geometry is not compatible with potential field data. Four sub-regions displayed a large discrepancy between calculated and observed data. This study redefines the subsurface geometries of these sub-regions through individual geometry inversions. The density and magnetic susceptibility ranges of units are further refined through property inversions. The modified geometry of the Devonian granites in the four sub-regions may be summarized as follows: 1) the Housetop Granite is relatively thin (≤5 km thickness), whereas 2) the Heemskirk and Meredith Granites are very thick and granite extends to a shallower depth between these bodies than previously interpreted. This region between plutons is thus a more prospective region than previously thought. 3) For the first time, an intrusive body underlying the eastern part of the Rocky Cape Group has been identified. Its petrophysical properties are similar to that of a granite, and its top is interpreted at a depth of >3 km. This interpreted low density (granitic) unit may be either Neoproterozoic or Devonian. 4) A new non-magnetic, low density Cambrian granite, with a minimum burial depth of 1 km, is also modelled in 3D, within the Mount Read Volcanics, in the south of the study area. Our approach, whereby sub-regions are identified for more detailed modelling, enables new constraints to be introduced in a computationally efficient way, and has general application to refining the geometry of key structures in prospective regions.

An improved adaptive neuro-fuzzy inference system using probability trajectory-based clustering ensemble

Research findings established the usefulness of Adaptive neuro-fuzzy inference system (ANFIS) in predicting outcomes of events, processes or systems from their numerical representations (datasets). However, its effectiveness in terms of prediction accuracy and complexity depends on some factors specifically its set of rules that models data behavior, and tune-able parameters (antecedent, consequent and rule base). Although several promising non-heuristic and meta-heuristic optimization techniques like gradient descent, Ant Colony, Genetic Algorithm, Particle Swarm, and Invasive Weed have been proposed to improve on the prediction accuracy of ANFIS through proper optimization of its tune-able parameters, but absolutely no work has applied clustering ensemble to improving on ANFIS for better accuracy especially on rules improvement. Therefore, in this paper, we propose an improved ANFIS that uses an agglomerative-based clustering ensemble of fuzzy c-means to help extract rules from a given dataset that represent a process, event or system. The ensemble uses probability trajectories of random walk processes on base clustering partitions of the dataset so as to refine direct co-association relationships or links among data objects of the base partitions thereby improving on these relationships that later metamorphosed to fuzzy rules and finally, a better accuracy. Comparative analysis of our proposed improved ANFIS alongside with the conventional ANFIS using two standard petrophysical datasets for lithology prediction shows that the proposed ANFIS is better than the conventional ANFIS in terms of prediction model accuracy and stability although, both maintain the same architecture in terms of the number of rules and tune-able parameters as well as the training methods.

Recent Advances in Geomathematics in Croatia: Examples from Subsurface Geological Mapping and Biostatistics

Geomathematics is extremely important in geosciences, particularly in the geology. The key for any geomathematical analysis is the definition of a typical model to be applied for further prognosis, either through deterministic or stochastic approaches. The selection of the appropriate procedure is presented in this paper. Two different geomathematical subfield datasets were used in subsurface geological mapping and palaeontology and different biostatistics applications, representing important geomathematical subfields in the Croatian geology. The different subsurface interpolation methods tested, validated and recommended for application were used to obtain the best possible outcome in reservoir modelling, in the cases with small datasets. Cross-validation may be chosen as the main selection criteria, applied to the Croatian part of the Pannonian Basin System (CPBS). Recent advances in biostatistics applied in palaeontology and case studies from Croatia are also presented, where biometric studies are of significant importance in fossil biota. Data, methods and problems in geosciences are vast subjects, and address a wide spectrum of fundamental science. Because geology includes subsurface and surface geology, and very different datasets regarding variable and number of data, we have chosen here two representative case study groups with original samples from Northern Croatia. Subsurface mapping has been presented on limited petrophysical datasets from the Northern Croatian, Miocene, hydrocarbon reservoirs. Biostatistics have been presented on very different samples, allowing us to achieve paleoenvironmental reconstructions of the size of relevant fossils, such as dinosaurs or other species and their paleoenvironments. All examples highlight examples of the valuable application of geomathematical tools in geology. The results, cautiously validated and correlated with other, non-numerical (indicator, categorical) geological knowledge, are of enormous assistance in creating better geological models.

An integrated analysis of mineralogical and microstructural characteristics and petrophysical properties of carbonate rocks in the lower Indus Basin, Pakistan

Abstract Carbonate rocks are believed to be proven hydrocarbon reservoirs and are found in various basins of Pakistan including Lower Indus Basin. The carbonate rock intervals of the Jakkher Group from Paleocene to Oligocene age are distributed in south-western part of Lower Indus Basin of Pakistan. However, there are limited published petrophysical data sets on these carbonate rocks and are essential for field development and risk reduction. To fill this knowledge gap, this study is mainly established to collect the comprehensive high quality data sets on petrophysical properties of carbonate rocks along with their mineralogy and microstructure. Additionally, the study assesses the impact of diagenesis on quality of the unconventional tight carbonate resources. Experimental techniques include Scanning Electronic Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray diffraction (XRD), photomicrography, Helium porosity and steady state gas permeability. Results revealed that the porosity was in range of 2.12 to 8.5% with an average value of 4.5% and the permeability was ranging from 0.013 to 5.8mD. Thin section study, SEM-EDS, and XRD analyses revealed that the samples mostly contain carbon (C), calcium (Ca), and magnesium (Mg) as dominant elemental components.The main carbonate components observed were calcite, dolomite, micrite, Ferron mud, bioclasts and intermixes of clay minerals and cementing materials. The analysis shows that: 1) the permeability and porosity cross plot, the permeability and slippage factor values cross plots appears to be scattered, which showed weaker correlation that was the reflection of carbonate rock heterogeneity. 2) The permeability and clay mineralogy cross plots have resulted in poor correlation in these carbonate samples. 3) Several diagenetic processes had influenced the quality of carbonates of Jakkher Group, such as pore dissolution, calcification, cementation, and compaction. 4) Reservoir quality was mainly affected by inter-mixing of clay, cementation, presence of micrite muds, grain compactions, and overburden stresses that all lead these carbonate reservoirs to ultra-tight reservoirs and are considered to be of very poor quality. 5) SEM and thin section observations shows incidence of micro-fractures and pore dissolution tended to improve reservoir quality.

Petrophysics and mineral exploration: a workflow for data analysis and a new interpretation framework

ABSTRACTAs mineral exploration seeks deeper targets, there will be a greater reliance on geophysical data and a better understanding of the geological meaning of the responses will be required, and this must be achieved with less geological control from drilling. Also, exploring based on the mineral system concept requires particular understanding of geophysical responses associated with altered rocks. Where petrophysical datasets of adequate sample size and measurement quality are available, physical properties show complex variations, reflecting the combined effects of various geological processes. Large datasets, analysed as populations, are required to understand the variations. We recommend the display of petrophysical data as frequency histograms because the nature of the data distribution is easily seen with this form of display. A petrophysical dataset commonly contains a combination of overlapping sub‐populations, influenced by different geological factors. To understand the geological controls on physical properties in hard rock environments, it is necessary to analyse the petrophysical data not only in terms of the properties of different rock types. It is also necessary to consider the effects of processes such as alteration, weathering, metamorphism and strain, and variables such as porosity and stratigraphy. To address this complexity requires that much more supporting geological information be acquired than in current practice. The widespread availability of field portable instruments means quantitative geochemical and mineralogical data can now be readily acquired, making it unnecessary to rely primarily on categorical rock classification schemes. The petrophysical data can be combined with geochemical, petrological and mineralogical data to derive explanations for observed physical property variations based not only on rigorous rock classification methods, but also in combination with quantitative estimates of alteration and weathering. To understand how geological processes will affect different physical properties, it is useful to define three end‐member forms of behaviour. Bulk behaviour depends on the physical properties of the dominant mineral components. Density and, to a lesser extent, seismic velocity show such behaviour. Grain and texture behaviour occur when minor components of the rock are the dominate controls on its physical properties. Grain size and shape control grain properties, and for texture properties the relative positions of these grains are also important. Magnetic and electrical properties behave in this fashion. Thinking in terms of how geological processes change the key characteristics of the major and minor mineralogical components allows the resulting changes in physical properties to be understood and anticipated.

Petrophysical “Big Data” - case study from the Stavely Project, western Victoria

SummaryThe application of recent technological advances to the acquisition of petrophysical datasets has significantly increased the amount of quality, systematic physical rock property measurements that can be captured efficiently and cost effectively. These large datasets provide a more comprehensive understanding of petrophysical values and, as importantly, their statistical distribution – particularly when encompassing a diverse set of rock types and stratigraphy. To date, these types of comprehensive datasets are limited in the mineral exploration space, in both number and their spatial distribution.This paper presents a case study from the Stavely Project area in western Victoria outlining the workflow from acquisition, compilation and analysis, to the application of a large, pre-competitive petrophysical dataset. These data, including gamma density, magnetic susceptibility, P-wave velocity, electrical resistivity and natural gamma, were acquired by scanning of diamond core from 21 stratigraphic and mineral exploration drill holes by Multi-Sensor Core Logger equipment, providing a combined final dataset of 216,699 petrophysical values. The precompetitive data and subsequent analyses are available for download.In developing a statistical analysis of the petrophysical values, the datasets were segregated into stratigraphic, lithological and spatial sub-populations, therefore giving a geological context for the data, and providing baseline values for regional geophysical interpretations, forward modelling and inversions, and the identification of regions of anomalous petrophysical values potentially associated with hydrothermal mineralisation. Crosscorrelation of various petrophysical properties highlights important relationships within the data, thus maximising its value. In western Victoria these data have been important in the delineation of prospective volcanic belts of the Stavely Arc beneath cover, and the development of a 3D model displaying the regional geometries and distributions of the belts at depth.

Petroleum systems of the Proterozoic in northwest Queensland and a description of various play types

As part of Queensland Government’s Strategic Resources Exploration Program, in conjunction with the Australian Government’s Exploring for the Future program, a study to improve the subsurface knowledge of Proterozoic basins in northwest Queensland (NWQ) is underway. Proterozoic sedimentary basins are prevalent across central and western Australia. Several of these basins have proven petroleum systems, with the best discoveries to date being in the Greater McArthur Basin, Northern Territory. Recent exploration and appraisal in the Beetaloo Sub-basin of the Greater McArthur Basin has identified large volumes of gas resources contained within unconventional shale reservoirs. In NWQ, the Isa Superbasin and overlying South Nicholson Basin are related in both age and likely deposition to the Greater McArthur Basin. The thick, extensive shale units of the Isa Superbasin are excellent source rocks, while the Mullera Formation in the South Nicholson Basin also has potential but has not been investigated in detail. There are several potential reservoirs within the Proterozoic section and younger units of the overlying Georgina and Carpentaria basins, including clastic and carbonate types. Exploration in the Isa Superbasin identified an estimated 22.1 trillion cubic feet of prospective resources (Armour Energy 2015) in unconventional shale reservoirs of the Lawn Hill Formation and Riversleigh Siltstone. This paper will discuss the stratigraphy, depositional and structural history of these Proterozoic basins and characterise their source and reservoir units using existing and recently acquired geophysical, geochemical, petrographic and petrophysical datasets. From this, several plays or play concepts will be identified and described to help understand the region’s potential for both conventional and unconventional petroleum resources.

Geomechanical characterization of mud volcanoes using P-wave velocity datasets

Abstract Mud volcanoes occur in many petroliferous basins and are associated with significant drilling hazards. To illustrate the type of information that can be extracted from limited petrophysical datasets in such geomechanically complex settings, we use P-wave velocity data to calculate the mechanical properties and stresses on a two-dimensional vertical section across a mud volcano in the Azeri-Chirag-Guneshly field, South Caspian Basin. We find that: (1) the values of the properties and stresses calculated in this way have realistic magnitudes; (2) the calculated pore fluid pressures show spatial variations around the mud volcano that potentially highlight areas of fluid recharging after the most recent eruption; and (3) the information obtained is sufficient to provide helpful indications of the width of the drilling window. Although calculations of this kind may be readily improved with more sophisticated petrophysical datasets, the simplicity of this approach makes it attractive for reconnaissance surveys designed to identify targets worthy of further investigation in developing our understanding of mud volcano geomechanics, or which could be used to help formulate drilling strategies.

Fracture patterns and petrophysical properties of carbonates undergoing regional folding: A case study from Kurdistan, N Iraq

The Zagros-Taurus fold and thrust belt hosts a prolific hydrocarbon system. Most hydrocarbon reserves are stored in naturally fractured reservoirs and such fracture systems can therefore have a significant impact on reservoir performance. Fractures are one of the most important paths for fluid flow in carbonate reservoirs, and industrial geoscientists and engineers therefore need to understand and study fracture patterns in order to optimise hydrocarbon production. The observed fracture patterns in outcrops may have implications on fluid flow and reservoir modelling in subsurface reservoirs, and we have therefore undertaken a case study of fracturing associated with regional folding in Iraqi Kurdistan. In this area, some exploration wells currently target Upper Triassic dolostones (Kurra Chine Formation) and/or Lower Jurassic limestones and dolomitised limestones (Sehkaniyan Formation). In both units hydrocarbon production comes mainly from secondary porosity created by dolomitisation, dissolution and fracturing. Both formations have undergone multiple phases of deformation associated with burial, uplift, folding and thrusting. We investigate some fracture pattern characteristics and some petrophysical properties of these units using selected outcrops around the Gara, Ora and Ranya anticlines that form folds directly traceable for 25–70 km. Our outcrop data is compared with subsurface fracture and petrophysical datasets reported from wells in the nearby Shaikhan and Swara Tika Fields. The 1-2-3D fracture attributes collected from outcrops are fracture orientation, type, spacing, intensity, length and cross-cutting and abutting relationships. Fracture orientations show a clear relationship to the local fold axis in both the outcrop and subsurface, although in some cases they appear to relate more to the present day in-situ maximum horizontal stress direction or local strike-slip faulting. Three stages of fracturing are proposed: pre-folding, early-folding and post-folding fractures. In addition, we report petrophysical properties - porosity, permeability and acoustic velocity of both the Kurra Chine and Sehkaniyan formations in relation to their structural position within folds and faults and stratigraphic level. The highest porosities and permeabilities are recorded in the hinges and backlimbs of the Gara Anticline. The best reservoir quality (highest porosity and permeability) is often found in areas associated with replacement dolomite i.e. solution vugs and intercrystalline porosity. The Kurra Chine Formation displays similar trends in velocity-porosity data at both outcrop and the subsurface. However, the Sehkaniyan Formation displays lower acoustic velocity for a given porosity at outcrop compared to the subsurface.

Automatic classification of carbonate rocks permeability from 1H NMR relaxation data

The accurate permeability mapping, even with the aid of modern borehole geophysics methods, is still a big challenge on the reservoir management framework. One concern within the petrophysics community is that rock permeability value predicted by well logging should not be considered as absolute, mainly for carbonates, but a relative index for identifying more permeable zones. Therefore, in this paper a permeability classification methodology, based exclusively on 1H NMR (Nuclear Magnetic Resonance) relaxation data, was evaluated for the first time as an alternative to the prediction of permeability as a continuous variable. To pursue this, a side-by-side comparison of different data mining techniques for the permeability classification task was performed using a petrophysical dataset with 78 rock samples from six different carbonate reservoirs. The effectiveness of six classification algorithms (k-NN, Naïve Bayes, C4.5, SMO, Random Forest and Multilayer Perceptron) was evaluated to predict the rock permeability class according to the following ranges: low (<1mD), fair (1–10mD), good (10–100mD) and excellent (>100mD). Discretization and feature selection strategies were also employed as preprocessing steps in order to improve the classification accuracy. For the studied dataset, the results demonstrated that the Random Forest and SMO strategies delivered the best classification performance among the selected classifiers. The computational experiments also evidenced that our approach led to more accurate predictions when compared with two methods widely adopted by the petroleum industry (Kenyon and Timur–Coates models).

Porosity gain and loss in unconventional reservoirs: Example of rock typing in Lower Cretaceous hemipelagic limestones, SE France (Provence)

We characterize porosity evolution in hemipelagic limestones of the Lower Cretaceous Vocontian Basin (Northern Provence, SE France), which are surface analogs for deep unconventional reservoirs. Firstly, a structural and sedimentologic analysis was conducted on outcrops and on borehole cores of Berriasian to Lower Valanginian rocks. Secondly, 330 plugs were analyzed for facies, pore typing, porosity and acoustic Vp velocity measurements. This petrophysical dataset was compared to the burial-uplift history of the host rock.Hemipelagic carbonates include four sedimentary formations comprising nine facies splitted into mud- and grain-dominated formations which display 0–8% porosity range depending on the initial grain to mud ratio of the sediments and on basin stress regime evolution during burial-uplift history. Initial porosity was better preserved in grain dominated formations where early micritic cements built bridges between grains, generating a solid skeleton that prevented/inhibited the burial compaction effects. In mud dominated facies, micrite was initially diffuse within the rock matrix but was progressively rearranged under burial stress, while the generation of an extensive amount of stylolites occurred. The differential response of compacted grain and mud dominated facies to a late regional 1 km uplift resulted in large porosity generation (should range from 15 to 35%) localized in the mud dominated facies through the dilatancy of stylolite and pressure-solution seams.This study highlights how important is the combination of classical approaches including facies, pore type and diagenesis characterization with the structural analyses. One key result applicable to several types of unconventional reservoirs is the effective porosity generated by reactivation of compaction structures.

Definitive Petrophysical Evaluation of Thin Hydrocarbon Reservoir Sequences

SummaryThe petrophysical evaluation of thinly-bedded sand/shale sequences is strongly dependent on the way in which layered and dispersed shales are taken into account. There are two underpinning issues: the recognition of thin bed geometries and the estimation of shale content from lithology logs. The first of these is managed through a scenario approach, whereby different downhole scenarios are set out according to specified ranges of bed thickness. Each range has its own interpretation protocol. The second issue relates to the inherent overestimation of clay-mineral volume fraction by shale indicators and its impact on the petrophysical evaluation of thinly-bedded sand porosity. This shortcoming is managed by applying a validated scaling factor to log-derived estimates of shale volume fraction so that these become workable estimates of clay-mineral fraction for the porosity evaluation of sand layers. Both these problems become more acute for thinner beds. These two key issues are addressed conjunctively through interpretive workflows, each of which relates to a discrete range of bed thickness and uses fit-for-purpose petrophysical data sets. In particular, the multicomponent induction log is adopted as a pivotal technology for the evaluation of sand resistivity within thinly-bedded sand/shale sequences. The objective is to achieve a sound petrophysical interpretation for each downhole situation. Therefore, groundtruthing through core data is advocated where this allows the shale problem to be managed better. The outcome is a systematic approach to thin-bed evaluation, based on prescribed data inputs that optimize project effectiveness. The benefits are illustrated through case histories that demonstrate a securable upside in projected hydrocarbon volumes.