Core Description Data Research Articles

Transformation of dense shelf water cascade into turbidity currents: Insights from high-resolution geophysical datasets

Dense shelf water cascade (DSWC) is a common oceanographic phenomenon on many continental shelves. Previous studies indicate that the DSWC could shape seabed physiography and carry seawater, sediment, and organic carbon a long distance from the continental shelf to the basin floor. However, it remains enigmatic how these DSWC's interact with seabed geomorphology and travel long distances from the shallow to deep marine environments. In this study, we employed high-resolution multibeam bathymetry, 2D and 3D seismic reflection, core description, and sediment grain size data from the Gippsland Basin, southeast offshore Australia. The continental shelf of the central Gippsland Basin stores sediment supplied by the along-shelf transported DSWC. By calculating the sediment motion threshold, we demonstrate that the DSWC is capable of entraining sediment from, and forming dense bottom nepheloid layers above, the seabed. Seismic reflection data reveal that cyclic steps are common on the shelf and slope, indicating a downslope-transported, supercritical current-dominated environment. Core observation and grain size analyses reveal that coarse-grained, Ta-typed turbidites are the major facies, indicating the presence of high-intensity downslope-traversing turbidity currents. Thus, supercritical turbidity currents are the dominant sedimentary process in the central Gippsland Basin. We illuminate that DSWC can interact with pre-existing seabed bathymetry created by a buried submarine landslide, resuspending sediment and igniting downslope-transported turbidity currents. The presence of numerous cyclic steps indicates that the turbidity current can evolve into a supercritical regime upon ignition, leaving complex seabed geomorphology and allowing the forming currents to travel across the shelf and extend more than 80 km down the lower slope. As revealed by our literature review, we imply that the transformation of DSWC into turbidity currents should be a common sedimentary process on outer continental shelves globally, significantly sculpting the seabed morphology and facilitating sediment and other marine particles transportation from shallow to deep sea.

Sedimentary systems of the Oligocene Huagang Formation in the central anticline zone of the Xihu Depression, East China Sea Shelf Basin

The Xihu Depression is the largest hydrocarbon-bearing depression of the East China Sea Shelf Basin (also referred to as the ECSSB). However, the depositional systems and reservoir distribution of the Oligocene Huagang Formation in the Xihu Depression are still controversial. Under the guidance of sedimentology and stratigraphy, this study documented a marine-terrestrial transitional environment in the restricted bay setting of the Oligocene Huagang Formation through core description, well logging, and seismic data analysis. This study also revealed that the Oligocene Huagang Formation is dominated by tidal delta, estuary, and gravity flow deposits in the central anticline zone of the Xihu Depression. The new understanding of the sedimentary systems and the discovery of the transgressive gap in the eastern Diaoyu Islands uplift explain the origin of fine-grained sediments and the EW-trending sand bodies in the central depression and the sand bodies parallel to shoreline in the west slope belt, which cannot be explained by previous study results, such as southern transgression or fluvial deltas and even lacustrine deposition. Moreover, the tidal channels, tidal sand flats, and gravity flow sand bodies formed by the transgressive tides are high-quality reservoirs. The study will provide a basis for well placement and serve as guidance for the selection of favorable hydrocarbon exploration areas in the Xihu Depression.

Lithofacies discrimination of the Ordovician unconventional gas-bearing tight sandstone reservoirs using a subtractive fuzzy clustering algorithm applied on the well log data: Illizi Basin, the Algerian Sahara

The main objective of this paper is to prove the capability of the fuzzy clustering algorithm for discriminating between lithofacies that are derived from the borehole log data of Ordovician tight sandstone reservoirs in the Illizi Basin of the Algerian South-Eastern Sahara. The technique is based on applying an unsupervised machine learning fuzzy algorithm on the natural gamma-ray, the photoelectric factor, the sonic log, the neutron porosity, and the bulk density well log data as an input to discriminate lithology into four lithofacies types: clay, sand, clayey sand, and sandy clay. The primary advantage of the fuzzy algorithms is that they can handle uncertainties and errors associated with the inaccurate or incomplete data of any measurements. Furthermore, it has a broader scope and a higher level of generality than the binary logic, which means that the results and interpretations that are based on this approach can be more appropriate and probable than other possible interpretations. A comparison between our algorithm's results and the results from the Kohonen Self-organizing Maps (SOM) and the Multilayer Perceptron (MPL) has also been carried out which proved that our approach provides better results than the other commonly applied neural networks. The obtained lithofacies types are highly comparable with that obtained from the core description data with minimal estimated errors. Therefore, this fuzzy algorithm can be applied as an alternative, fast, and low-cost technique for adequate lithofacies discrimination in case of lacking the core data.

Research on Sedimentary Microfacies of an Oil Layer in a Block of Songliao Basin

This topic uses debris, core log, core description data, combines regional research results, determine that the oil layer in a block of the Songliao Basin is the most developed in the front subfacies and prodelta subfacies of delta underwater sediment. Microfacies include underwater distributary channels, underwater natural levees, distributary bays, estuary dams, distal bars, sheet sand, and prodelta mud. The contiguous distribution of dam sand is the main feature of this area. On the basis of single-well facies research, it is the first time that this topic has studied the features of the plane and profile distribution of the reservoir sedimentary facies belt in the study area, and finally builds the sedimentary facies model, and provides a solid geological basis for fine reservoir description and favorable area prediction.

Skin factor and potential formation damage from chemical and mechanical processes in a naturally fractured carbonate aquifer with implications to CO2 sequestration

In this study, we investigate formation damage due to acidization and water injection tests into the naturally fractured carbonate Middle Duperow Formation at Kevin Dome, Montana, potentially diminishing the chance of a future successful Geological Carbon Sequestration (GCS) project. Multiple well-test analytical models, correlated with core description and lithology data, are used to determine flow behavior and communication between the water injection interval and surrounding formations. An improved three-dimensional (3D) geologic model with dual-continuum matrix and fracture properties is constructed based on most recent seismic, core, and water sample measurements. Brine injection is simulated to verify the interpretation from the analytical models, followed by CO2 injection simulation. Geochemical calculations are performed to understand the in-situ processes that led to formation damage. Our findings suggest: (1) there are two possible scenarios that could lead to a positive total effective skin factor and permeability decline: partial penetration and formation damage; (2) analytical models indicate a positive total skin factor, contradicting results of a previous study suggesting that the well was mildly stimulated; (3) numerical simulation supports the formation damage hypothesis by matching the pressure buildup observed during the latter two brine injection tests; (4) several mechanical and chemical processes may have occurred during injection to clog the matrix/fracture system: anhydrite fines migration and/or calcite precipitation. We then make preventative suggestions for future GCS projects into carbonate reservoirs and remediation recommendations for GCS operation at the Kevin Dome.

Key exploration & development technologies and engineering practice of continental shale oil: A case study of Member 2 of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin, East China

Abstract Based on detailed core description and systematic joint test data, enrichment laws of continental shale oil have been examined deeply. Key technologies such as the identification and quantitative evaluation method for sweet spot, precise design and tracking of horizontal well trajectory, and the low-cost horizontal well volume fracturing technology of the whole process “slick water + quartz sand” for continental shale oil have been formed. The research results show that the enrichment of pure continental shale oil of the Paleogene Kong 2 Member in Cangdong Sag is controlled by predominant fabric facies and cross-over effect of retained hydrocarbons jointly; and there are four modes of shale oil enrichment, i.e. laminar felsic, laminar mixed, thin-layer limy dolomitic, and thick-layer limy dolomitic shales. The identification and evaluation method for shale oil sweet spots can predict sweet spots accurately. The precise trajectory design for sweet spot layer and tracking-trajectory optimization while drilling by considering geological and engineering factors have been proved effective by field application, with drilling rate of sweet spots reaching 100% and drilling rate of type I sweet spots reaching over 75%. The whole process “slick water + quartz sand” low cost volume fracturing has been proved effective in creating multi-stage fracture network in the horizontal section, and improved productivity greatly. It can lower the comprehensive engineering cost by 26.4%.

Application of Multi-Resolution Graph-based Clustering for electrofacies prediction: a case study from the Horn River Shale, British Columbia, Canada

The Horn River Basin is a major shale gas play in British Columbia, Canada. An important component for identifying productive zones is to determine lithofacies. Quantitative core-based lithofacies classifications distinguish reservoirs and non-reservoir quality zones. In this study, Elemental capture spectroscopy (ECS) log data, XRD, and TOC data were integrated with core-description data to define the shale gas development interval in the Horn River Shale. Six lithofacies were determined and used as a training data for the supervised electrofacies classification. The Multi-Resolution Graph-based Clustering method (MRGC) was useful for prediction of electrofacies. The MRGC facilitated estimation of the electrofacies from the well logs of the non-coring wells based on our current construction model. We constructed a supervised MRGC model from well A. The probability scores of the electrofacies were 83%. This method was applied to non-coring wells B and C. Lateral variations in the facies were inferred using an electrofacies correlation or seismic-scale spatial model. Stochastic methods were applied to build a 3D facies model due to the insufficient electrofacies data. To increase the accuracy and applicability we used seismically derived-trend data (density and P-wave sonic inversion, envelope attributes) for sequential indicator simulation (SIS) modeling. After constructing a facies model, the faintly laminated siliceous mudstone (FLSM) and homogeneous siliceous mudstone (HSM) facies were shown to be dominantly distributed throughout the Evie Member and Muskwa Formation. These facies contain high silica compared to clay and higher TOC contents which are generally the main targets of a shale gas play. Therefore, the FLSM and HSM facies of the Muskwa Formation and Evie Member are potentially productive facies for shale gas development in the Horn River Shale.

Facies classification and patterns of lacustrine carbonate deposition of the Barra Velha Formation, Santos Basin, Brazilian Pre-salt

Abstract The lacustrine carbonate rocks of the giant Pre-salt oil-fields in the Santos and Campos Basins differ in a number of important aspects from classic lake deposits. Prior work has described distinctive and unusual features of deposition, early diagenesis and reservoir quality with in these rocks that defy classification using established schemes. In addition to their unusual textures, debate continues as to how to interpret the vertical stacking pattern of facies. A new facies classification scheme is proposed based on the relative abundance of three end-member components: mud (micrite and clay minerals), calcite spherulites and fascicular calcite shrubs. Although focused on the main textural and mineralogical features of the rocks as seen today, the proposed scheme takes into account the relative contribution of depositional and early diagenetic components, sediment mineralogy and the degree of dissolution. Application of this scheme offers insight into depositional conditions, whilst also linking to reservoir quality. Basic (elementary) cycles can be defined using this classification in combination with the variation in the abundance of detrital grains and the chemical composition. Three possible models are suggested to interpret the origin of the basic cycle within the Barra Velha Formation in the context of variation in climate, lake level and water chemistry. Integration of data from different sources, including petrographic, core description and chemical data for the basic cycle, offers improved process-based understanding of metre-scale facies patterns and provides a foundation for understanding larger scale vertical variations within and between unconformity-bound sequences.

Characterization of the Lower Silurian Longmaxi marine shale in Changning area in the south Sichuan Basin, China

On the basis of the core description, thin sections, and geochemistry data, we characterized the Lower Silurian Longmaxi marine shale in the Changning area in the south Sichuan Basin. Ten lithofacies were identified in the Lower Silurian Longmaxi marine shale: (a) massive silty shale (Lithofacies A); (b) laminated siliceous calcareous shale (Lithofacies B); (c) calcareous shale (Lithofacies C); (d) silty dolomitic shale (Lithofacies D); (e) argillaceous siltstone (Lithofacies E); (f) siltstone (Lithofacies F); (g) laminated silty shale (Lithofacies G); (h) silty calcareous shale (Lithofacies H); (i) argillaceous lime shale (marl) (Lithofacies I); and (j) bentonite (Lithofacies J). Obvious heterogeneity occurred in different lithofacies in terms of mineralogy, total organic carbon (TOC) content, porosity, permeability, pore surface area, total pore volume, and total gas content. Among these lithofacies, Lithofacies B has the highest porosity (5.4%), pore surface area (12.79 m2/g), total pore volume (0.015 cc/g), TOC content (4.37%), total gas content (3.4 m3/ton), and relatively high brittle minerals, suggesting great shale gas potential. Lithofacies A, C, and D have a relatively high brittle minerals, porosity, pore surface area, total pore volume, TOC content, and total gas content, which means that these 3 lithofacies show relatively good potential for shale gas. The rest of the lithofacies have low brittle minerals content, porosity, pore surface area, total pore volume, TOC content, and total gas content, representing poor shale gas potential.

Sequence stratigraphic analysis of fluvial deposits using facies characterization and wireline log correlation: case of the late Early-early Middle Devonian Snake Cave Interval, Darling Basin, Australia

In this article, we present the application of wireline log correlation for sequence stratigraphic analysis to the late Early to early Middle Devonian Snake Cave Interval in the Neckarboo Sub-basin (DM Kewell East DDH-1 well) and Hillston Trough (DM Mossgiel DDH-1 well), Darling Basin. The results of an integrated study using characterising wireline log signatures (gamma ray, sonic, resistivity shallow, long spaced neutron and limest neutron porosity) and core description data supplemented by limited palaeontological reports indicate that three major upper Lower to lower Middle Devonian cycles have controlled sedimentation of this stratigraphic succession. We distinguished seven core sedimentary facies, which were grouped into six electrofacies defined by wireline log signatures. The electrofacies were defined by describing and analysing the graphic gamma ray logs using basic geometrical shapes, such as bell, serrated bell, funnel, serrated funnel, cylinder (block) and linear. Six electrofacies informally named electrofacies SA, SB, SC, SD, SE and SF correspond to seven core facies (MS1-MS7), which have progressively greater sandstone/shale ratios and better electrofacies quality. These electrofacies are characterised as follows: electrofacies SA corresponds to the conglomeratic braided channel-fill, electrofacies SB corresponds to channel lag and point bar, electrofacies SC corresponds to crevasse splay sandstones and flood-plain fines, electrofacies SD corresponds to multistorey fluvial channel-fills, electrofacies SE corresponds to sandy braided channel-fills and electrofacies SF corresponds to distributary channel sands and local small delta-plain. The upper Lower to lower Middle Devonian sequence (Snake Cave Interval) of the Neckarboo Sub-basin and Hillston Trough is characterised by two major third-order sequences (Snake Cave sequence 1 and Snake Cave sequence 2) that consist of ten systems tracts and ten sequence surfaces. Snake Cave sequence 1 (SCS1) consists of two cycles with progradational to retrogradational trends (LST1 to TST1 and LST2 to TST2 separated by a marine flooding surface) divided into five sequence surfaces (SBSC1, TS1 MFS1, TS2 and MFS2, in ascending order) as defined by wireline log signatures. One cycle has progradational, retrogradational to aggradational trends (LST3, TST3 to HST1) and has seven sequence surfaces (e.g. MFS2, LST3, TS3, MFS3 and SBSC2, in ascending order). Snake Cave sequence 2 (SCS2) consists of one unit with progradational, retrogradational to aggradational trends (LST4, TST4 and HST2, in ascending order) with three sequence surfaces (e.g. SBSC2, TS4 and MFS4). No TST4 and HST packages were observed, and the MFS4 is poorly developed. The recognition of sedimentological sequence stratigraphic changes above and below marine flooding surfaces (MFS1-4), where deposition sequences have been defined without adequate biostratigraphic control, is easy since depositional sequence and bounding surface correlation can always be clearly demonstrated. A sequence stratigraphic architecture that can be used to accentuate and strengthen Darling basin exploration also documents significant changes in the local and regional tectonostratigraphic setting.

Integrated Petrophysical Evaluation of a Heterogeneous Shaly-sand Reservoir: A Case Study in Nahr Umr Formation-subba Giant Oil Field, Iraq

This study presents an integrated petrophysical evaluation for Nahr Umr reservoir by highlighting a practical trustworthy approach petrophysical model for Nahr Umr reservoir in Subba oil field depending on available data. The available data include core analysis conducted on seven core samples taken from seven wells in Subba oil field and well set logs which include gamma ray, static spontaneous, neutron, sonic, density and resistivity logs. Subba oil field is a giant oil field located in southern part of Iraq about 110 km Northwest Basra. Nahr Umr formation in Subba oil field is a heterogeneous clastic reservoir. It was considered one of the most important producing clastic reservoir in southern Iraqi oil fields. The conventional petrophysical models, by using neutron- density cross-plot for lithology and porosity and Archie's equation for water saturation, can be adequate for evaluating clean sands. Usually the shaly sand formations are adhere to bad hole in shale section. Facies identification was achieved depending on core analysis data, core description data and wire line log measurements. The calculations of permeability was based on rock types identification and using available core data measurements. The generated permeability models can be used to calculate permeability for uncored intervals in all wells corresponding to each facies. Porosity was calculated using Neutron-density porosity model and Sonic model for bad hole section. Saturation model was built by utilizing the Indonesia Equation model, this model has proved its efficiency to be applied successfully for clastic Iraqi reservoirs. The petrophysical parameters were determined on a well-by-well basis. The formation resistivity factors (a, m) and water saturation exponent (n) have been derived from Pickett plot and formation water salinity. Reservoir quality calculations or net pay values are based on cut-offs values for porosity, saturation and shale volume.

Evaluation of oil sands resources —A case study in the Athabasca Oil Sands, NE Alberta, Canada

Oil sands are the most important of the oil and gas resources in Canada. So the distribution and evaluation of oil sands form a critical basis for risk investment in Canada. Distribution of oil sands resources is severely controlled by the reservoir heterogeneity. Deterministic modeling is commonly used to solve the heterogeneity problems in the reservoir, but rarely used to evaluate hydrocarbon resources. In this paper, a lithofacies based deterministic method is employed to assess the oil sands resources for a part of a mining project in northern Alberta. The statistical analysis of Dean Stark water and oil saturation data and study of the core description data, regional geology and geophysical logs reveal that the lithofacies in the study area can be classified into reservoir fades, possible reservoir fades and non-reservoir fades. The indicator krigging method is used to build a 3D lithofacies model based on the classification of sedimentary fades and the ordinary krigging method is applied to petrophysical property modeling. The results show that the krigging estimation is one of the good choices in oil sand resources modeling in Alberta. Lithofacies-grade based modeling may have advantages over the grade-only based modeling.

Sequence lithofacies paleogeography and reservoir potential of the Maokou Formation in Sichuan Basin

Abstract Based on multidisciplinary analyses of outcrops, core description and log data, two types of sequence boundary are identified, including a partially exposed unconformity surface and a drowned unconformity surface. The Maokou Formation in the Sichuan Basin is divided into three third-order sequences, each lasting 3.3 Ma on average. In this study, system tracts are used as mapping unit to reconstruct systematically the lithofacies paleogeography map within the sequence framework, and to describe paleogeographic features. The results indicate that the Sichuan Basin is dominated by shallow, open marine carbonate platform deposits during the Middle Permian Maokou stage, in which a range of sedimentary facies developed, such as shallow water open platform, deeper water open platform, platform margin bank, and platform front slope facies. Physical property analysis indicates that shallow calcsparite granular limestone has good reservoir qualities, especially after the reservoir rock is reworked by dolomitization and epigenetic karstification. Thus, dolomite reservoir, paleokarst reservoir and shallow bank reservoir make up the main reservoirs of the Maokou Formation. The study on reservoir distribution patterns within the sequence framework indicates that HST reservoirs are relatively well developed, especially the shallow bank sedimentary regions of SQ2-HST in the western-southwestern Sichuan Basin and the paleokarst development area of SQ3 are the most favorable.

A Fully Integrated Method for Dynamic Rock Type Characterization Development in One of Iranian Off-Shore Oil Reservoir

Rock selection in modeling and simulation studies is usually based on two techniques; routinely defined rock types and those defined by special core analysis (SCAL). The challenge in utilizing these two techniques is that they are frequently assumed to be the same, but in practice, static rock-types (routinely defined) are not always representative of dynamic rock-types (SCAL defined) in the real reservoir. There is also no significant link between these two techniques. To fill this gap, we integrate the well log data for identification of the optimal number of rock-types, and SCAL data with its high interpretive potential in a given reservoir zonation. In this paper, we propose a method in one of Iranian offshore oil reservoir with a tight carbonate formation for dynamic rock type characterization. In this method, with the integration of well logs and core description data using multivariate statistical methods, different static rock-types can be identified, but these rock types cannot be assigned for fluid flow simulation. So, with our approach based on capillary pressure curves, different flow behavior can be classified. This technique can be done by using integration of similar capillary pressure curves due to the inlet pressure corresponding to the log parameters. Finally, with integration of capillary pressure and well log data, two different dynamic rock-types with distinct flow behavior were identified. This method can be used for the development of rock-type characterization and deriving of saturation height functions for calculation of initial water saturation in any heterogonous reservoir and it is an applicable solution for inputs in Geomodel and also simulation models.

Orbital-driven cyclicity and the role of halokinesis on accommodation within siliciclastic to carbonate, shallow-water Albian deposits in the Espírito Santo Basin, southeastern Brazil

The basal section of the lower Albian Barra Nova Group in the onshore portion of the Regência Platform, a central-eastern geological province of the Espírito Santo Basin, Brazil, comprises a mixed succession characterized by the cyclic alternation of dominant fluvial to shallow marine, siliciclastic rocks and marine carbonate strata. Spectral analysis of gamma-ray logs of four wells has shown consistent evidence of a short eccentricity orbital frequency (100 ka) influence on sedimentation. The spectral analysis points to a time span from 2.95 to 3 Ma (depending on the method employed) for the deposition of the interval studied in the most complete section (Well 1), and from 1.84 to 2.1 Ma in the least complete section (Well 4), owing to partial erosion. These figures are consistent with the palynological dating of the section (2 Ma), hence showing that cyclostratigraphy can play an important role in assigning a time span for high-frequency cycles (4th and 5th orders). Climatic factors not necessarily linked to eustasy, such as the alternation of dry (carbonate deposition) and mild humid (increased siliciclastic input) climate, are supposed to have controlled the cyclical stratigraphic arrangement. Accommodation as a result of diastrophism, loading and eustatic sea-level rise would not explain such a high mean accumulation rate (29.1 cm/ka) estimated through cyclostratigraphy. Therefore, based on these assumptions and seismic data, the high preservation potential, and resulting well-preserved cyclicity, is herein assigned to halokinesis. Several pieces of evidence obtained from the analysis of seismic data, core description, stratigraphic correlation, palaeontology and spectral data have shown the importance of halokinesis to increasing accommodation, and then the excellent preservation of the cyclical nature of the geological record. This is unusual in sedimentary environments subject to recurrent erosive events such as those related to shallow-marine settings.

A fully integrated approach for the development of rock type characterization, in a middle east giant carbonate reservoir

Rock typing in modelling and simulation studies is usually based on two techniques; routinely defined rock types and those defined by special core analysis (SCAL). The challenge in these techniques is that it is frequently assumed that these two are the same to assign, but in practice static rock-types (routinely defined) are not always representative of dynamic rock-types (SCAL defined) in the reservoir. There is also no significant link between these two. To fill this gap, we propose a method for the combination of core description data with well log data for identification of the optimal number of static rock-types, and SCAL data with its high interpretive potential for identifying effective dynamic rock-types within determined static rock-types in a given reservoir zonation. In this paper, the proposed method was applied for two new exploration wells in one of the southern carbonate reservoirs in Iran with the lowest number of available data. First, with the integration of well logs and core description data using multivariate statistical methods, seven static rock-types were identified. Next, in these static rock-types the values of the permeability and reservoir quality index were predicted using neural networks with matching of core and log data. Then, based on the correlation of static rock-types and reservoir quality index a reservoir zonation consisting of nine zones was determined. Finally, with interpretation of SCAL data in the different positions of static rock-types for each zone, six different dynamic rock-types with distinct flow behaviour above the oil water contact were identified. This method can be used for the development of rock-type characterization in any heterogonous reservoir and it is a workable solution as input for saturation functions to the simulation model.

Neural network prediction of carbonate lithofacies from well logs, Big Bow and Sand Arroyo Creek fields, Southwest Kansas

In the Hugoton Embayment of southwestern Kansas, St. Louis Limestone reservoirs have relatively low recovery efficiencies, attributed to the heterogeneous nature of the oolitic deposits. This study establishes quantitative relationships between digital well logs and core description data, and applies these relationships in a probabilistic sense to predict lithofacies in 90 uncored wells across the Big Bow and Sand Arroyo Creek fields. In 10 wells, a single hidden-layer neural network based on digital well logs and core described lithofacies of the limestone depositional texture was used to train and establish a non-linear relationship between lithofacies assignments from detailed core descriptions and selected log curves. Neural network models were optimized by selecting six predictor variables and automated cross-validation with neural network parameters and then used to predict lithofacies on the whole data set of the 2023 half-foot intervals from the 10 cored wells with the selected network size of 35 and a damping parameter of 0.01. Predicted lithofacies results compared to actual lithofacies displays absolute accuracies of 70.37–90.82%. Incorporating adjoining lithofacies, within-one lithofacies improves accuracy slightly (93.72%). Digital logs from uncored wells were batch processed to predict lithofacies and probabilities related to each lithofacies at half-foot resolution corresponding to log units. The results were used to construct interpolated cross-sections and useful depositional patterns of St. Louis lithofacies were illustrated, e.g., the concentration of oolitic deposits (including lithofacies 5 and 6) along local highs and the relative dominance of quartz-rich carbonate grainstone (lithofacies 1) in the zones A and B of the St. Louis Limestone. Neural network techniques are applicable to other complex reservoirs, in which facies geometry and distribution are the key factors controlling heterogeneity and distribution of rock properties. Future work involves extension of the neural network to predict reservoir properties, and construction of three-dimensional geo-models.

SEA LEVEL CHANGE AND THE DEPOSITIONAL HISTORY OF A SHALLOW, COASTAL AQUIFER SYSTEM, NORTHERN MONTEREY, CA, BASED ON INDICATOR GEOSTATISTICS

We conducted a multidisciplinary investigation of shallow aquifers and confining layers within a coastal groundwater basin in northern Monterey County, CA. Shallow sediments within this basin comprise a Holocene, unconfined, aeolian and valley-fill aquifer, and an underlying, Pleistocene aquifer of fluvial and alluvial silt, sand, and gravel. These two aquifers are separated by a confining unit of clay, silt, and sand, previously interpreted to be a lagoonal and shallow marine deposit of latest Pleistocene to Holocene age. The 40 km2 study area contains 231 wells and borings, from which cuttings and core description data were digitized and compiled at 0.6-m depth intervals. Additional geophysical logging (natural gamma-ray and induction resistivity) data was collected in 55 wells. This information was used for proxy lithological identification based on calibration from core descriptions, and combined with the core and cuttings data to generate a database of lithological interpretations at 22,000 X-Y-Z locations. These data were used to generate geostatistical realizations, using a binary (clay versus not clay) indicator scheme for the confining unit and shallow, valley-fill aquifers. The realizations indicate a complicated, three-dimensional geometry that is not apparent in earlier cross-sections of the confining unit, and appears to record multiple sealevel changes, most likely associated with isotope Stage 5. This interpretation has important implications for the hydrogeology of this site and the Salinas Valley in general, as well as other coastal California basins.

High Resolution Reservoir Architecture of Late Jurassic Haynesville Ramp Carbonates in the Gladewater Field, East Texas Salt Basin: ABSTRACT

The East Texas Salt Basin contains numerous gas fields within Upper Jurassic Haynesville ramp-complex reservoirs. A sequenced-keyed, high-resolution zonation scheme was developed for the Haynesville Formation in Gladewater field by integrating core description, well-log, seismic, porosity and permeability data. The Haynesville at Gladewater represents a high-energy ramp system, localized on paleotopographic highs induced by diapirism of Callovian Age Salt (Louann). Ramp crest grainstones serve as reservoirs. We have mapped the distribution of reservoir facies within a hierarchy of upward-shallowing parasequences grouped into low-frequency sequences. The vertical stacking patterns of parasequences and sequences reflect the interplay of eustasy, sediment accumulation patterns, and local subsidence (including salt movement and compaction). In this study we draw on regional relations from analogous, Jurassic systems in Mexico to constrain the stratigraphic architecture, age model, and facies model. Additionally, salt-cored Holocene, grain-rich shoals from the Persian Gulf provide excellent facies analogs. The result is a new high-resolution analysis of reservoir architecture at a parasequence scale that links reservoir facies to depositional facies. The new stratigraphy scheme demonstrates that different geographic portions of the field have markedly distinct reservoir intervals, both in terms of total pay and the sequence-stratigraphic interval within which it occurs. Results from thismore » study are used to evaluate infill drill well potential, in well planning, for updating reservoir models, and in refining field reserve estimates.« less

Using a Microcomputer for Entry of Geologic Core Descriptions: ABSTRACT

Sohio realized the need for computer manipulation of lithologic data from a large number of cored wells. The specific goals were (1) comparing lithologic observations with foot-by-foot log measurements, and (2) rapid generation of lithologic maps. These goals required the creation of a consistent core description data base. Traditional geologic core descriptions, where many feet of rock are often summarized in a written paragraph, were found unsuitable for incorporation into a data base. Core description data sheets were also found unsatisfactory because of the time and cost required to locate and correct errors inherent in these forms. With the Sohio Digital Entry of Core Description (SODECD), there is immediate data entry into a microcomputer, thereby providing excellent quality control. An assistant types the data into the microcomputer as the geologist describes the core. Features of the software include: (1) prompts for each data category, (2) cursor will skip past prompts not relevant, (3) allows movement forward and backward through prompts, (4) displays menus of valid responses, (5) completes quality control checks and displays error messages when invalid data is entered, (6) saves each foot of core description on diskette, and (7) allows the core description data to be transferred to the main computer and incorporated into the data base. Compared to using core description data sheets, SODECD was found a more rapid and less expensive method of creating a core description data base. This data base is more accurate and consistent because the following problems are eliminated: (1) invalid data entries, (2) data entered in wrong column, (3) omission of significant data categories, (4) null versus zero ambiguity, (5) geologic inconsistencies, and (6) keypunching cost and errors. The incorporation of core descriptions into a data base allows a variety of computer analysis, such as rapid production of lithologic maps, plots of lithologic data along with log measurements, and computer-produced stratigraphic columns. End_of_Article - Last_Page 532------------