INTERPRETATION OF GALVANIC RESISTIVITY, ELECTROMAGNETIC INDUCTION, AND DIELECTRIC DISPERSION LOGS TO ESTIMATE WATER SATURATION IN ORGANIC RICH SHALE

Water saturation estimation based on electrical and electromagnetic logs in organic-rich shale reservoirs is challenging due to effects of clay and conductive minerals. Presence of nanopores, low porosity, interfacial polarization effects, and highly anisotropic tortuous paths in organic-rich shales and tight hydrocarbon-bearing formations results in breakdown of conventional models for purposes of water saturation estimation. These petrophysical attributes lead to disagreement between water saturation estimates obtained using different approaches, such as Nuclear Magnetic Resonance (NMR), Induction, Dielectric, and Dean-Stark retort measurement method. Few of the above-mentioned challenges in water saturation estimation can be addressed by jointly processing the subsurface galvanic resistivity, induction, propagation and dielectric dispersion logs using an integrated mechanistic model that accounts for various frequency-dependent interfacial polarization mechanisms. Single-frequency galvanic resistivity (laterolog) or single-frequency induction resistivity and multi-frequency dielectric dispersion logs, comprising 4 conductivity and 4 dielectric permittivity logs measured at four distinct frequencies were acquired in few wellbores drilled in Wolfcamp and Bakken shale formations. We implement a novel log-interpretation technique for the improved estimation of water saturation (S_w), brine conductivity (C_w), textural index/cementation exponent (m), and saturation exponent (n). The proposed log-processing workflow uses an integrated mechanistic model, which combines Complex Refractive Index (CRI) model to analyze the conductivity and permittivity logs acquired at 1 GHz, Stroud-Milton-De (SMD) model to analyze the 3 conductivity dispersion and 3 permittivity dispersion logs in the frequency range of 10 MHz to 0.3 GHz, and Waxman-Smits (WS) model to analyze the single-frequency deep galvanic resistivity log (RLA5) acquired at 1 kHz or the single-frequency induction resistivity (AT90) log acquired at 20 kHz. Petrophysical estimates derived from the joint inversion were robust in the presence of pyrite, low water saturation, and low porosity as compared to the petrophysical estimates from the inversion of only four-frequency dielectric dispersion logs or those from the single-frequency resistivity log. Based on the petrophysical estimates from the proposed methodology, formation brine conductivity and saturation-exponent estimates are more reliable compared to water saturation and cementation exponent estimates. Water saturation estimates obtained using the proposed methodology are compared against those obtained using multi-mineral inversion and CRIM model and those from NMR log, Dean-Stark core measurements and service company’s dielectric inversion. Average relative errors in fitting the 1 laterolog resistivity and 8 dielectric dispersion logs using the estimates obtained from the proposed method are 10% and 20%, respectively, and their extreme values are 55% and 60%, respectively, in the 520-ft depth interval of the upper Wolfcamp shale formation. Average relative errors in matching the 1 induction resistivity and 8 dielectric dispersion logs using the inversion-derived estimates are 33% and 20%, respectively, in the 350-ft depth interval of Bakken Petroleum System (BPS). Water saturation and formation brine conductivity estimates in Middle Bakken are in the ranges of 0.5 to 1 and 25 S/m to 45 S/m, respectively. In the Upper Wolfcamp formation, water saturation estimates are in the range of 0.4 to 1 and connate water conductivity are in the range of 5 S/m to 30 S/m

Borehole Geophysics Applied to the Evaluation of Groundwater Monitoring Well Construction and Integrity

Geolog

A multidisciplinary geophysical approach to recognize and quantify the gas occurrence in the Northern Adriatic

L'Adriatico settentrionale \ue8 caratterizzato da una presenza diffusa di gas nel sottosuolo, testimoniata da vari segni distintivi sul fondo mare e nei sedimenti sottostanti. Nel 2009 e nel 2014, OGS ha effettuato nell'area due campagne di sismica a riflessione 2D , finalizzate ad indagare la relazione tra le emissioni di gas, i cammini di migrazione e l'assetto geologico della regione. Inoltre, si voleva caratterizzare da un punto petrofisico i fluidi all'interno della successione sedimentaria. In questa tesi, due linee sismiche perpendicolari del dataset sono state selezionate ed analizzate. L'obiettivo era la determinazione della distribuzione del gas e la stima della sua concentrazione lungo le due linee sismiche, all'interno della successione sedimentaria Plio-Quaternaria e correlando log di pozzo e attributi sismici. Un approccio multidisciplinare \ue8 stato applicato sia nell'analisi che nell'interpretazione dei dati, integrando l'informazione geologica e quella geofisica. La caratterizzazione del gas nell'area di studio ha due implicazioni importanti: 1) Una valutazione complessiva della presenza di gas e della sua dinamica di migrazione all'interno della successione sedimentaria pu\uf2 vincolare il ruolo delle strutture tetttoniche identificate 2) L'approccio innovativo e multidisciplinare usato pu\uf2 essere applicato in altre aree di studio, caratterizzate dalla presenza di gas nei sedimenti e in simili assetti geologici. Inoltre, considerato il fondale molto basso dell'area, il trasferimento di gas dai sedimenti alla colonna d'acqua e nell'atmosfera potrebbe essere rapido e significativo. Comprendere a fondo i meccanismi di fuoriuscita di gas ha quindi importanti implicazioni climatologiche, dato che il metano, il componente principale del gas nell'area, ha un ruolo di spicco come gas serra.The Northern Adriatic is characterized by widespread occurrence of gas in the subsurface, testified by the presence of fluid related seabed and sub-seabed features (Donda et al., 2015, and references therein). In 2009 and 2014 OGS carried out two 2D multichannel seismic surveys in the area to constrain the relationship between gas emissions, migration paths and the regional geological setting. A further aim was to characterize the gas-charged fluids occurring within the sedimentary succession. Two seismic lines from these datasets were analyzed in the framework of this PhD thesis (GEA project, \u2019Gas Emissions in the Northern Adriatic Sea\u2019). The objective of this thesis is to determine the distribution of gas and estimate its concentration along the two selected perpendicular seismic profiles within the Plio-Quaternary sedimentary succession, through the petrophysical characterization of the sediments with the use of well-log to seismic attributes correlation. A multidisciplinary approach was applied both in the analysis and in the interpretation of the data, through the integration of geological information with geophysical wireline logs and seismic data. The proper characterization of the gas occurrence in the study area has two direct implications: 1. The comprehensive evaluation of both the gas occurrence and its migration within the sedimentary succession will constrain the role of the identified tectonic features; 2. The innovative and multidisciplinary approach used in this thesis could be applied to other areas characterized by gas-charged fluids in similar geological settings. Moreover, due to the shallow water in the Northern Adriatic, the potential transfer of gas from sediment to the water column and then into the atmosphere could be significant. Therefore, understanding the mechanisms of natural gas seepage has important climatological implications, since methane, the main component of the gas in the area (Donda et al., accepted), plays a major role as a greenhouse gas

Integrating Petrophysics and Allostratigraphy to Find Sweet Spots in the Upper Cretaceous Belle Fourche and Second White Specks Alloformations, West-Central Alberta, Canada

The Upper Cretaceous Second White Specks Formation – an organic-rich, calcareous mudrock succession within the lower Colorado Group – is a prolific self-sourcing tight oil reservoir in Alberta. Due to the low porosity and permeability of this interval, localized natural fracture networks have previously provided the only means for oil to flow at economic rates. This study, focused in west-central Alberta, used allostratigraphic methods to subdivide the Second White Specks Formation into allomembers that define hydraulic flow units. The petrophysical properties (porosity, organic content, clay volume, and brittleness) of each allomember were modelled using a basic suite of geophysical wireline logs and sparse core data. When overlain with historic oil production results, modelled petrophysical parameters delineated previously unrecognized “sweet spots” that likely have increased oil potential. Applying the innovative petrophysical workflow developed in this study may increase the likelihood of drilling successful wells in similar exploration scenarios with limited datasets

Sedimentary, geochemical and geophysical study of the Ecca group, Karoo supergroup and its hydrocarbon potential in the Eastern Cape Province, South Africa

The Ecca Group of Karoo Supergroup is a sedimentary rock sequence that deposited between the Late Carboniferous (Dwyka Group) and the Late Permian-Middle Triassic (Beaufort Group). The Ecca Group investigated in this study is situated in the Eastern Cape Province of South Africa and it comprises mainly of shales, mudstones, siltstones and sandstones. The Ecca Group sequence contains considerable carbon content and suitable thickness to make it an ideal target for shale gas exploration. Previous studies put more emphasis on the geology and stratigraphy of the Ecca Group, this study revised the stratigraphy, and put new insight on the petrography, depositional processes, sedimentary facies, provenance, paleoweathering, tectonic setting, subsidence rates and history, electrical resistivity, source rock characteristics and diagenesis of the potentially feasible sandstone and mudrock reservoir rocks of the Ecca Group. Based on the lithological features, sedimentary structures and facies characteristics, the stratigraphy of the Prince Albert, Whitehill, Collingham and Fort Brown Formations of the Ecca Group is now subdivided into two informal members each, i.e. Lower Member and Upper Member. Furthermore, the Ripon Formation is now subdivided into three informal members. Each member has been asigned a lithological name. The grain size parameters show that most of the Ecca Group sandstones are very fine to fine grained, poorly to moderately well sorted, mostly near-symmetrical and mesokurtic in grain-size distribution. The linear discriminant function analysis is dominantly indicative of turbidity current deposits under deep marine environment for Prince Albert, Whitehill and Collingham Formations, shallow marine environment for Ripon Formation, while the Fort Brown Formation is lacustrine-deltaic deposits. Modal composition analysis and petrography studies revealed that the detrital components of the sandstones are dominated by monocrystalline quartz, feldspar and lithic fragments. The sandstones are compositionally and texturally immature and can be classified as feldspathic wacke and lithic wacke. The provenance analysis revealed plutonic and metamorphic terrains as the main source rocks with minor debris derived from recycled sedimentary rocks. The detrital modal compositions of these sandstones are related to back arc to island and continental margin of tectonic setting. Based on the detailed sedimentological analyses of outcrop and borehole data, fourteen lithofacies were identified and seven facies associations (FAs) were recognised. The facies associations are: FA 1: Shale and mudstones intercalated with siltstones, FA 2: Carbonaceous shale, mudstone with subordinate chert and sandstone, FA 3: Mudstones rhythmite with thin bedded mudstone and lenticular siltstone, FA 4: Greyish medium bedded sandstone intercalated with laminated mudstone, FA 5: Dark-grey medium to thick bedded mudstone and siltstone, FA 6: Thin to medium bedded sandstone alternated with thin bedded carbonaceous mudstone, and FA 7: Varved mudstone rhythmite intercalated with siltstone and minor sandstone. Sedimentological characteristics of the identified facies associations indicate four deposition environments, namely, deep marine basin, turbidite, shallow marine and lacustrine environments, which constitute a gradually regression sequence as a result of sea-level dropping and shallowing of the basin during the developmental processes. Geochemical analysis of the Ecca mudrocks and sandstones revealed that the rocks are of quartzose sedimentary provenance, suggesting that they were derived from a cratonic interior or recycled orogen. The petrography and geochemistry of the sandstones indicated that the source areas are composed of plutonic and metamorphic rocks with a minor component from sedimentary rocks. The geochemical diagrams and indices of weathering suggested that the granitic source rocks underwent moderate to high degree of chemical weathering. The tectonic setting discrimination diagrams support passive continental margin setting of the provenance