Bogdan NICULESCU | University of Bucharest (original) (raw)

Papers by Bogdan NICULESCU

Proceedings of 22nd International Multidisciplinary Scientific GeoConference SGEM 2022 (ISBN 978-619-7603-38-5, ISSN 1314-2704), Jul 11, 2022

Romula, the most important urban settlement of Dacia Inferior (Dacia Malvensis) province of the R... more Romula, the most important urban settlement of Dacia Inferior (Dacia Malvensis) province of the Roman empire during the 2nd and 3rd centuries A.D., was located in the present-day Reşca village—Dobrosloveni commune, Olt County, Romania. Most of the Roman town remains are underneath Reşca village premises, the areas suitable for archaeological or geophysical research being limited. This paper presents an integrated geophysical study carried out in the Central Fort zone of the archaeological site in September 2020, which included magnetic, gravity, and geoelectrical (ERT—Electrical Resistivity Tomography) surveys. The magnetic survey covered the entire Central Fort area (3 hectares), in continuous data acquisition mode, using profiles with approximately N-S orientation and 2 m spacing between the profiles. The gravity and ERT surveys were conducted on profiles with NE–SW and NW–SE orientation, with 2 m spacing between successive gravity measurements and 0.8 or 1 m electrode spacing. The geophysical data analysis revealed distinct anomalous zones and lineaments with preferential NW–SE and NE–SW orientations, likely corresponding to Roman construction elements incorporating baked bricks or stone, brick debris, pits, or furnaces. An anomalous lineament possibly corresponding to an underground sewer or drainage channel was also detected in the northern extremity of the surveyed perimeter. These preliminary results demonstrate the applicability of the selected geophysical investigation methods in the specific conditions of Reşca–Romula archeological site. A future research direction consists of testing the validity of the geophysical survey results through systematic archaeological excavations.

Energies, Jan 15, 2024

The Late Jurassic–Early Cretaceous (J3–K1) transboundary aquifer is the most important groundwate... more The Late Jurassic–Early Cretaceous (J3–K1) transboundary aquifer is the most important groundwater body from southern–southeastern Romania, shared with Bulgaria and hosted in karstic–fractured carbonates. We conducted an integrated evaluation of this aquifer by analyzing three 700-m-deep groundwater exploration–exploitation boreholes which intercepted it in Cernavodă area (South Dobrogea region). The evaluation was based on geophysical wireline logging, drilling information, and borehole production tests. A K-means clustering of the logging data was performed for lithology typing, formation boundaries identification and delineation of probable producing intervals associated with secondary porosity development. Petrophysical interpretation was carried out via depth-constrained (zonal) inversion, using multimineral models, the estimated formation boundaries, and variable uncertainties for the main input logs. The optimal interpretation models were correlated with borehole testing results, to gain insight into the hydrogeological properties of the aquifer complex. The fractured–vuggy interval with highest water-producing potential was identified in the lower section of the J3-age Rasova Formation (639–700 m depth) comprising mainly undolomitized limestones. A southeast-to-northwest trend of increasing productivity of the boreholes, correlated with increasing lateral dolomitization intensity within the Rasova Formation, suggests a highly heterogeneous character of the aquifer. The differences in productivity are due not only to local porosity variations but also to various degrees of pore space connectivity related to the amount of fracturing or karstification. The novel findings of this study have important practical implications for the optimal placement, design, and drilling program of future groundwater exploitation boreholes in Cernavodă area and neighboring sectors.

Conference Proceedings (EarthDoc online database), EAGE 4th Asia Pacific Meeting on Near Surface Geoscience and Engineering, Nov 30, 2021

Integrated geophysical investigations including magnetometric, gravimetric, and 2D Electrical Res... more Integrated geophysical investigations including magnetometric, gravimetric, and 2D Electrical Resistivity Tomography (ERT) surveys were conducted in the Central Fort zone of the Roman archaeological site Romula (Reşca village, Olt County, Romania). This study, sponsored by the Foundation of the Society of Exploration Geophysicists (SEG)—Tulsa, OK, USA, was carried out by teams from the University of Bucharest, Faculty of Geology and Geophysics—Department of Geophysics, and from the Geological Institute of Romania—Geohazard Department. We aimed to assess the effectiveness of various geophysical methods under the specific conditions of this site and to provide new information regarding archaeological targets, in addition to the results of a previous ERT survey from 2019 and of the archaeological excavations. The collected magnetometric data show distinct anomalous zones or lineaments with NW–SE and NE–SW orientation, likely reflecting buried Roman structures (building remains, brick walls, brick debris); generally, the identified anomalous magnetic sources correlate well with high resistivity ERT anomalies. Some particularly significant results of the 2020 geophysical survey are the clear delineation of the northeastern brick wall of the Roman Central Fort and of a possible sewage or drainage channel outside the Fort perimeter.

Cronica Cercetărilor Arheologice din România: Campania 2020 (ISSN 2343-919X), Oct 31, 2021

Energies (eISSN 1996-1073), Oct 14, 2021

The successful interpretation of open-hole well logging data relies on jointly using all availabl... more The successful interpretation of open-hole well logging data relies on jointly using all
available petrophysical and geological information. This paper presents relevant case studies related to the integration of well logs with core measurements for exploration wells drilled in the Romanian continental shelf area of the Western Black Sea basin. The analyzed wells targeted gas-bearing sands and silts complexes of Early Pliocene (Dacian) age, developed in a deltaic to shallow marine sedimentary environment in two distinct fields. The wireline logging programs included conventional formation evaluation logs, pressure surveys, nuclear magnetic resonance, and borehole electrical imaging logs. The core dataset comprised routine and special measurements
(porosity, grain density, permeability, water saturation, and Archie parameters) carried out at quasi-reservoir confining pressure. The wireline logging suites were interpreted via a deterministic workflow, including core-derived interpretation parameters. Other core-derived parameters were used for constraining and validating the log interpretations. The results show that a problem related to the ambiguity of formation water resistivity can be overcome through resistivity–porosity dependencies constructed to include potential aquifer zones in the proximity of the Dacian gas-bearing reservoirs. This study also revealed and quantified uncertainties regarding the
estimation of gas–water contacts from formation pressure surveys, which can be mitigated by the confirmation or correction of pressure-derived fluid contacts via the well log interpretation results. Lastly, we identified a probable resistivity logs suppression effect related both to high contents of capillary-bound water and also to the limited resolution of electrical logging tools in the presence of sand-shale thin bedding or laminations.

Proceedings of 20th Anniversary International Multidisciplinary Scientific GeoConference SGEM 2020 (ISBN 978-619-7603-05-7, ISSN 1314-2704), Sep 20, 2020

A major risk factor for freshwater coastal aquifers is represented by seawater intrusion, consist... more A major risk factor for freshwater coastal aquifers is represented by seawater intrusion, consisting of the movement of marine saltwater into these aquifers. This occurs due to sea level changes, tidal fluctuations, changes in evaporation and recharge rates, fractures in coastal rock formations, or excessive freshwater pumping. The negative effects of this salinization phenomenon are a reduction in the available freshwater storage volume, the contamination, and the abandonment of production wells.
In Romania, such intrusion has occurred in the southern part of Black Sea's coastline, in Costineşti and Vama Veche resorts, affecting the main aquifers hosted in late Middle Miocene limestones.
In Costineşti area, Vertical Electrical Sounding (VES) surveys were carried out repeatedly between 1991 and 2010, along a 900 m length WNW-ESE profile on the lineament of several water exploitation wells and pumping stations. These surveys identified and monitored a significant minimum resistivity anomaly associated with a seawater intrusion, at about 2000 m distance from the coastline, generated by the overexploitation of drinking water. The reduction of freshwater exploitation led to diminishing contamination and gradual disappearance of the associated resistivity anomaly.
In Vama Veche area the geoelectrical researches were initiated in 2009, with 23 VES surveys performed on two parallel profiles of about 800 m length each and with WSW–ENE orientation. This investigation revealed on the southern profile a minimum resistivity anomaly associated with a seawater intrusion at 45 m average depth, advancing at least 150–200 m inland. The researches were restarted in 2019, in the framework of a Field Camp supported by the Society of Exploration Geophysicists. A number of 8 profiles with N–S, WE , NNE–SSW and WNW–ESE orientation and a total length of 1800 m were imaged via 2D Electrical Resistivity Tomography (ERT). These ERT surveys indicated that the seawater intrusion is more extended than initially considered, reaching at least 500 m distance from the coastline. They also allowed the identification of potential intrusion pathways, represented by a system of fractures or faults with an approximate NW–SE/WNW–ESE and, possibly, N–S orientation.

Acta Geophysica (ISSN 1895-6572, eISSN 1895-7455), Feb 27, 2021

Acta Geophysica (ISSN 1895-6572, eISSN 1895-7455), Jan 21, 2021

Following previous geoelectrical researches initiated in 2009 for the delineation and characteriz... more Following previous geoelectrical researches initiated in 2009 for the delineation and characterization of seawater intrusion in a freshwater aquifer of Sarmatian (late Middle Miocene) age, a subsurface imaging survey via 2-D electrical resistivity tomography (ERT) was conducted in 2019 on the outskirts of Vama Veche resort—Romanian Black Sea southern coast. The survey was organized in the framework of a field camp sponsored by the Foundation of the Society of Exploration Geophysicists (SEG)–Tulsa, OK, USA, with participation of teams from the University of Bucharest—Department of Geophysics and the Geological Institute of Romania. A number of eight ERT profiles with N–S, W–E, NNE–SSW, and WNW–ESE orientation and 155–315 m length were imaged with a SuperSting R8/IP instrument (Advanced Geosciences Inc.), using deployments of 32–64 electrodes at 5 m spacing, in Wenner, Schlumberger, and dipole–dipole array configurations. The processing and interpretation of high-resolution ERT data indicated that the seawater intrusion, evidenced as very low resistivity (5–10 Ω m) anomalous zones starting at approximately 45–49 m depth, has advanced at least 500 m inland. The survey results also allowed the identification of a system of fractures or faults with an approximate NW–SE/WNW–ESE and, possibly, N–S orientation, that might have provided potential pathways for saline water intrusion.

E3S Web of Conferences (eISSN 2267-1242), Volume 133 (2019), Article 01001, Nov 25, 2019

This study presents the results of the structural and sedimentological interpretation of borehole... more This study presents the results of the structural and sedimentological interpretation of borehole electrical imaging data recorded in a gas exploration well from the Romanian offshore area of western Black Sea. The well intercepted on the investigated interval Pliocene to Late Miocene deposits, including a gas reservoir hosted in Early Pliocene shallow marine sands and silts. A total of 824 features were picked on the electrical image, such as bedding, soft sediment deformations, open (conductive) fractures and closed (resistive) fractures. An analysis of the dip angles and azimuths data identified four structural domains within the study interval: two are quasi-horizontal, one shows slightly inclined bedding dips and one is dominantly deformed (slump features and possibly sediment creep). The open fractures show a preferential WNW–ESE strike, whereas the closed fractures show, besides a WNW–ESE strike, mostly ENE–WSW to N–S strikes. The opposite directions of the two fracture sets suggest a relationship to the regional tectonic stress, with open fractures being nearly parallel to maximum horizontal stress SHmax direction and closed fractures being nearly normal to SHmax direction. The variable bioturbation intensity observed on the electrical image may reflect salinity fluctuations in the Black Sea basin.

Acta Geophysica (ISSN 1895-6572, eISSN 1895-7455), Aug 21, 2019

Seawater intrusions are a major environmental hazard for coastal freshwater aquifers. They are ge... more Seawater intrusions are a major environmental hazard for coastal freshwater aquifers. They are generated mainly by the uncontrolled exploitation of freshwater in pumping stations, if the aquifers are in hydraulic connection with the sea. In Romania, such marine intrusions have occurred in the southern part of Black Sea's coastline, in Costineşti and Vama Veche resorts, contaminating the main aquifers hosted in Sarmatian (late Middle Miocene) limestones, at distances ranging from hundreds of meters to over 2 km inland. For the study of these salinization phenomena, Vertical Electrical Sounding (VES) surveys were performed in the affected areas. These surveys allowed the delineation and spatial–temporal monitoring of the intrusions and offered information related to faults that may have provided pathways for seawater migration towards the exploitation wells. The 1D interpretation of VES apparent resistivity data was performed via a set of novel software applications. The forward modeling component of the applications uses digital linear filtering and allows the simulation of theoretical VES responses for horizontally-layered geological media with virtually unlimited number of layers. The pseudo-inversion component of the applications is based on a random sampling of the parameters space of the geoelectrical models. The interpretation of VES surveys recorded in Vama Veche area by using the elaborated software indicates that the seawater intrusion occurs at more than 40 m depth. This agrees with a well flow test which produced salt water at 40–60 m depth in that area.

Proceedings of 19th International Multidisciplinary Scientific GeoConference SGEM 2019 (ISBN 978-619-7408-77-5, ISSN 1314-2704), Jul 7, 2019

The identification of fluid contacts (gas–water contact—GWC, oil–water contact—OWC and gas–oil co... more The identification of fluid contacts (gas–water contact—GWC, oil–water contact—OWC and gas–oil contact—GOC) is essential for field reserve estimates and field development and, also, for detailed formation evaluation. For the accurate calculation of some petrophysical parameters, such as porosity, the reservoir interval has to be zoned by fluid type, to account for differences in fluid saturations and fluid properties (e.g., hydrogen index, density, sonic transit time) in the various intervals: gas cap, oil column and aquifer zone. The fluid contacts may vary over a reservoir either because of faults, semipermeable barriers, rock quality variations / reservoir heterogeneity, hydrocarbon-filling history or a hydrodynamic activity. Horizontal contacts are typically taken into consideration, although irregular or tilted contacts occur in some reservoirs. The methods used for determining the fluid contacts include fluid sampling, water and hydrocarbons saturation estimation from geophysical well logs, analyses of conventional or sidewall cores, and formation pressure measurements. The pressure profiles obtained with various formation testing tools over reservoir intervals are, frequently, the primary source of data for defining the fluid contacts. When good quality pressure data can be collected, the fluid contacts can be determined by identifying the depths at which the pressure gradients (pressure versus depth trends) change. This study addresses some issues related to the identification of GWC for two gas fields of Early Pliocene age (Dacian stage), belonging to the biogenic hydrocarbon system of western Black Sea basin-Romanian continental shelf. We show that the identification of these contacts based exclusively on pressure gradients analysis is uncertain or may be inaccurate. The pressure gradients approach should be checked against the results of the conventional interpretation of geophysical well logs (e.g. changes in the computed fluid saturations as a function of depth) and, if available, the results of nuclear magnetic resonance (NMR) log investigations, which are able to indicate the intervals with clay-bound water, capillary-bound water and movable fluids.

Proceedings of 19th International Multidisciplinary Scientific GeoConference SGEM 2019 (ISBN 978-619-7408-77-5, ISSN 1314-2704), Jul 7, 2019

Integrating conventional well log interpretation with high-resolution borehole electrical image a... more Integrating conventional well log interpretation with high-resolution borehole electrical image analysis enables a complex understanding of the geological formations intercepted by exploration wells. Whereas conventional geophysical well logs allow the determination of the main reservoir parameters and the fluids distribution, borehole imaging serves as a valuable tool in sedimentary features identification and interpretation, regional dip determination, structural and tectonic elements identification, as well as the analysis of borehole failures (such as breakouts and natural or drilling-induced fractures) related to tectonic stress in a particular area. In addition to conventional well log interpretation, we have carried out a detailed analysis of the borehole electrical imaging data for a gas exploration well drilled in Sarmatian (late Middle Miocene) deltaic deposits from northwestern Moldavian Platform, Romania. 1100 borehole features were picked and interpreted across a 196 m interval (386–582 m depth) covered by the borehole image. The number of picks provided a satisfactory statistical dataset to carry out a dip analysis, faults and fractures characterization and present day stress analysis. The features identified on the borehole image include bedding and soft-sediment deformations (in shales), cross-bedding (in sands and silty sands), faults, open fractures, borehole breakouts and unconformable surfaces. A statistical analysis of 775 low-angle planar features picked in shale intervals and representing bedding revealed two dominant dip directions towards SSW (200–220 degrees dip azimuth, 3.1 degrees dip angle modal value) and SSE (150 degrees dip azimuth, 7.2 degrees dip angle modal value), with subordinate preferential dip directions towards ENE or east (60–100 degrees azimuth). The dominant directions are consistent with the general structure of the Moldavian Platform, which dips from NE to SW and, also, towards south or SSE. Two high-angle faults (55–58 degrees formation dip angles) were recognized on the borehole image and they might have provided pathways for biogenic gas migration from deeper levels up to shallower Sarmatian sand reservoirs. Borehole breakouts were found on various intervals and they show a counterclockwise rotation of their azimuth (from 155 to 116 degrees) along with increasing depth, attributable to localized stress field perturbations resulted from faulting.

Proceedings of 19th International Multidisciplinary Scientific GeoConference SGEM 2019 (ISBN 978-619-7408-77-5, ISSN 1314-2704), Jul 7, 2019

Borehole instability issues account for approximately 18% of the non-productive time during drill... more Borehole instability issues account for approximately 18% of the non-productive time during drilling. The data needed for the planning of optimized wellbore trajectories and mud weight windows can come from multiple sources, notably Logging-While-Drilling (LWD) and wireline measurements. In particular, in-situ tectonic stresses can be inferred from the identification and characterization of breakouts (some of the most common processes that infringe upon the stability of boreholes). Two techniques can help to distinguish the breakouts: (1) determining borehole ovalization and (2) identifying the breakouts on borehole geophysical images. Multi-arm caliper logs from wireline imaging tools provide a direct measure of the borehole size but are not always available. Borehole shape can, however, be inferred from LWD geophysical imaging logs, notably from acoustic and density logs, which are very sensitive to standoff. Synthetic caliper curves can be computed from the standoff curves and converted into an oriented caliper image, displayed as a borehole wireframe. Sections through this borehole wireframe provide ovalization information and eliminate the uncertainty of breakouts direction estimation from resistivity images, which may show sometimes artifacts related to the presence of bad hole conditions or to very high formations true resistivity/mud resistivity ratios.
This paper presents the application of LWD imaging data for better characterization of a borehole cross-section and easier breakouts detection in hostile environments (borehole excavations and rugosity, resistivity image artifacts, presence of fractures and faults). The proposed method enables detecting breakouts in real time or in memory mode. Determining the magnitude and orientation of borehole breakouts enables the validation or adjustment of geomechanical models and the selection of optimal parameters for the drilling mud.

Revue Roumaine de Géophysique / Romanian Geophysical Journal (ISSN 1220-5303), Jan 1, 2016

We investigate the potential usefulness of Principal Component Analysis (PCA) method in providing... more We investigate the potential usefulness of Principal Component Analysis (PCA) method in providing meaningful petrophysical information, in addition to the results obtained via conventional well log interpretation, or to constrain and validate such results. We applied PCA to a geophysical logging data set recorded in a natural gas exploration well drilled in the NW part of Moldavian Platform-Romania. The first principal components of the data seem to respond to major lithological changes or shale/clay content variations, whereas the higher-order principal components most likely reflect fluid-related data variability, such as fluids type and/or volume. The results of this study suggest that PCA may successfully complement the standard log interpretation and formation evaluation methods.

E3S Web of Conferences (eISSN 2267-1242), Volume 66 (2018), Article 01005, Nov 26, 2018

Metamorphic processes, leading to mineralogical and structural changes of the rocks in response t... more Metamorphic processes, leading to mineralogical and structural changes of the rocks in response to physical (pressure, temperature) and chemical conditions, can be associated with the development of sin-metamorphic or post-metamorphic fractures. The post-metamorphic ones are directly related to tectonic stress. In addition, tectonic stress may cause the reorientation of minerals on a direction perpendicular to the stress direction, generating foliations (schistosity). This paper presents a structural analysis of the pre-Alpine metamorphic basement and its relations with the sedimentary cover by using geophysical data recorded in an exploration well located in the North Dobrogean Promontory (Romania). The analysis was based on quad-combo wireline logs, spectral gamma ray, sonic cross-dipole, and borehole electrical imaging data. The imaging analysis allowed the identification and characterization of metamorphic foliations, sedimentary bedding, natural fractures, and the determination of tectonic stress orientation. A brittleness index was computed by means of elastic parameters derived from density and sonic compressional and shear logs. Also, a fracture intensity characterization by using fracture area per volume of rock was conducted. The integration of geophysical well logging with mud logging and drilling data allowed us to carry out a detailed analysis of the metamorphic basement in the studied area.

E3S Web of Conferences (eISSN 2267-1242), Volume 66 (2018), Article 01004, Nov 26, 2018

This paper addresses some formation evaluation challenges and petrophysical particularities regar... more This paper addresses some formation evaluation challenges and petrophysical particularities regarding two gas fields of Early Pliocene age, belonging to the biogenic hydrocarbon system of Western Black Sea Basin-Romanian continental shelf. Although these structures are located at the same depth and only 15 km apart, the wells that intercepted the sands and silts gas-bearing reservoirs indicate an important lateral facies variation and different reservoir qualities. We analyzed and interpreted data from exploration and appraisal wells that targeted these reservoirs, showing that: (1) there is a limited radioactivity contrast between the reservoir and non-reservoir intervals, so a clay volume determination based solely on the gamma ray log is not practical; (2) the reservoirs are characterized by high capillary-bound water contents, leading sometimes to abnormally low resistivity readings; (3) an additional resistivity suppression might be caused by the limited vertical resolution of the electrical logging tools, in the presence of thinly laminated sand-shale intervals; (4) the identification of gas-water contacts based exclusively on pressure gradients may be inaccurate and should be checked against the results of conventional geophysical logs interpretation and of nuclear magnetic resonance logs, for delineating the intervals with bound water or with movable fluids.

Proceedings of 18th International Multidisciplinary Scientific GeoConference SGEM 2018 (ISBN 978-619-7408-38-6, ISSN 1314-2704), Jul 2, 2018

The study area is located in the northern and western parts of Moldavian Platform, the oldest pla... more The study area is located in the northern and western parts of Moldavian Platform, the oldest platform unit of the Romanian territory and representing the margin of the East European Platform. Two hydrocarbon systems are recognized in the Moldavian Platform: a thermogenic system of Paleozoic age and a biogenic system of Miocene age. The Miocene biogenic system comprises significant natural gas fields (including dry gas with more than 98% methane), reservoired especially in Sarmatian (late Middle Miocene) deposits, where suitable conditions for accumulation and sealing are encountered. The Sarmatian stage was marked by permanent changes of the sedimentary conditions, passing from a predominantly marine environment to a transitional one, of deltaic type with lacustrine-continental influences. The gas accumulations are usually hosted in sands/sandstones (observed as good seismic reflectors with continuous or discontinuous character) that pinch out forming lithostratigraphic traps. The sand beds or sand bodies formed during the deltaic construction, especially when they overlap and alternate with pelitic sequences, offer the most favorable settings for such accumulations.
The integrated analysis of recent geophysical well logs (conventional logs and high-resolution electrical imaging logs) and seismic reflection surveys, together with mud logging data and well flow test results, allow a better characterization of the Sarmatian deposits, particularly the gas reservoirs, from the study area. The correlation of three exploration wells along a NW–SE profile indicates that a low-energy, fine-grained depositional environment is developing towards SE, with a prevalence of claystones and with fewer sand reservoirs, if any. This may reflect a deltaic transition from distributary channels and mouth bar sands towards prodelta offshore silts and muds. The processed electrical imaging data recorded in the northernmost exploration well show two dominant dip azimuths (142 and, subordinately, 218 degrees) in the shale intervals. Most likely, these indicate NW to SE and NE to SW sediment paleotransport directions, related to seaward delta progradation. The electrical imaging results also reveal the presence of two high-angle faults (48–54 degrees dip values), which might have provided pathways for gas migration from deeper levels up to shallower Sarmatian reservoirs.

Proceedings of the 18th International Multidisciplinary Scientific GeoConference SGEM 2018 (ISBN 978-619-7408-35-5, ISSN 1314-2704), Jul 2, 2018

Carbonate reservoirs are important exploration targets from the perspective of hydrocarbon and, a... more Carbonate reservoirs are important exploration targets from the perspective of hydrocarbon and, also, groundwater reserves. In this respect, the most important aquifer system in Romania is located in South Dobrogea region, comprising two distinct aquifer complexes: a deeper one, mostly under pressure, hosted in fractured/fissured and karstified limestones and dolomites of Late Jurassic–Early Cretaceous age and a shallower one, mostly unconfined, hosted in Sarmatian (late Middle Miocene) lumachellic and oolitic limestones. The deeper aquifer is regional, confined on more than 60% of its extension, has a SW–NE general flow direction and discharges in Siutghiol Lake (Constanţa city area) and through submarine springs on the Black Sea continental shelf. The water flow occurs through fractures/fissures and dissolution voids within the carbonate rocks and, also, along fault planes.
Although a large number of wells were drilled in South Dobrogea for groundwater exploitation from the Late Jurassic–Early Cretaceous aquifer and, also, for groundwater quality monitoring, there are few published data and studies regarding the geophysical response and petrophysical features of these formations, in relation with their hydrogeological characteristics. The more recent drilling of three hydrogeological research boreholes in the proximity of Cernavodă Nuclear Power Plant (NPP) offered an opportunity to gain valuable information concerning the geological succession in this northern part of South Dobrogea, with special regard to the carbonate formations hosting the aquifer. The boreholes were drilled to a depth of 700 m, for the identification and exploitation of aquifer horizons, in order to provide a fresh water supply for the NPP units. Geological analyses on drill cuttings and cores and geophysical well logging were carried out, for a comprehensive characterization of the intercepted formations from a lithologic and stratigraphic viewpoint, the identification of fractures/fissures and caverns as potential water-producing zones and the evaluation of formation porosity. The geophysical logs interpretation showed that the most probable water-producing zones are located in the so-called Dolomitic Complex (Kimmeridgian–Tithonian) on the 620–700 m (particularly 650–700 m) depth interval, in carbonates with average porosities of 12–16% and locally reaching more than 40%.

Proceedings of 18th International Multidisciplinary Scientific GeoConference SGEM 2018 (ISBN 978-619-7408-35-5, ISSN 1314-2704), Jul 2, 2018

Salt water intrusions of marine origin are a major cause of contamination for coastal fresh water... more Salt water intrusions of marine origin are a major cause of contamination for coastal fresh water aquifers. They are generated mainly by the uncontrolled exploitation of fresh water in pumping stations from coastal areas. In Romania, such marine intrusions have occurred in the southern part of Black Sea's coastline, in the area of Vama Veche and Costineşti resorts, contaminating the main aquifers which are hosted within Sarmatian (late Middle Miocene) lumachelic and oolitic limestones. Fresh water exploitation management requires the identification, mapping and monitoring of marine intrusions. These are achievable by means of an adequate application of geoelectrical methods, because sea water intrusions lead to major variations of the aquifers resistivity. Apparent resistivity surveys such as electrical profiling and vertical electrical sounding (VES) are able to delineate the areal extent of these intrusions and to identify the approximate depth of the fresh water/salt water interface. Assuming the subsurface geology consists of horizontal and homogeneous layers, the VES apparent resistivity data can be interpreted in terms of layer thicknesses and true resistivities, i.e. a one-dimensional (1-D) geoelectrical model with a stepwise variation of true resistivity with depth. The optimal model is obtained by repeated forward modeling trials or through the inversion of measured apparent resistivities.
This paper presents an algorithm and a set of software applications for the forward modeling of VES apparent resistivity curves, by using digital linear filtering. The codes were elaborated in MATLAB programming environment and allow the calculation of the theoretical VES response for horizontally-layered geological media with virtually unlimited number of layers, including layers with a quasi-continuous resistivity variation. The latter option can be used to simulate particular hydrogeological situations regarding the sea water intrusions in fresh water aquifers, which often show a mixing/transition zone of variable salinity and resistivity. The interpretation of VES curves recorded in Vama Veche area by using the elaborated software indicates that the sea water intrusion occurs at more than 50 m depth. This agrees with water wells tests which confirm the presence of salt water at 40–60 m depth in that area.

AAPG Search and Discovery, Apr 4, 2017

Fracture zones are usually characterized by an increased fracture density and thus a higher secon... more Fracture zones are usually characterized by an increased fracture density and thus a higher secondary porosity. In the studied area (Potwar Basin, Northern Pakistan), the porosities derived from a neutron log and fracture apertures (obtained via-high resolution resistivity images) are very low, even if the resistivity images show a high fracture density cataclastic zone. This analysis refers to a tight carbonate reservoir located close to a major fault zone, with a high density of fractures that do not contribute to fluid flow due to their small apertures. The compressive, reverse faulting regime with active hanging walls led to the formation of the main hydrocarbon traps, these being laterally delimited by such faults. Borehole breakouts and drilling-induced fractures were used for tectonic stress evaluation. The dominant minimum stress direction identified in the analyzed wells is WSW - ENE, normal to the direction of drilling-induced fractures, as indicators of the maximum stress direction, which are oriented NNW - SSE. The orientation of drilling-enhanced fractures confirms the maximum stress direction as determined via drilling-induced fractures. The alignment of the major fault and the natural fractures, parallel to the minimum horizontal stress direction, suggests a compressional tectonic regime at the moment of their formation. The maximum tectonic stress, normal to the fractures direction, has contributed to apertures closure and secondary porosity reduction. This explains why in an area characterized by intense cataclasis, resulted from the fault proximity, porosity is less than 2% and fracture apertures are, generally, less than 0.5 mm. But the simultaneous presence of breakouts and drilling-induced fractures in the same well is an indicator of a strike-slip stress regime. One may conclude that a change in the tectonic stress regime took place, from a compressive to a strike-slip one. The major fault, which initially had a compressive character (dominantly vertical movement), later became a strike-slip one, with a dominantly horizontal component. Knowledge of the fractures' propagation direction with respect to maximum tectonic stress is extremely important for the potential design of horizontal wells. Experience has shown that production is highly correlated to fracture volume and connectivity in tight formations, implying the future successful economic development of this particular field lies in knowledge of the fracture propagation direction.