Cross-plot Analysis Research Articles

ANALISIS SIFAT FISIS PADA RESERVOAR BATUPASIR MENGGUNAKAN METODE SEISMIK INVERSI IMPEDANSI AKUSTIK (AI) DAN MULTIATRIBUT PADA LAPANGAN “MNF” CEKUNGAN BONAPARTE

Analysis of the physical properties of the sandstone reservoir in this study was carried out using the acoustic impedance inversion method and seismic multi-attribute which was carried out at the Bonaparte Basin "MNF" Field. In this study, the acoustic impedance distribution is generated, and the acoustic impedance is derived to obtain a water saturation distribution map and the volume of the log property is predicted to predict pseudo gamma ray, density and porosity with multi-attribute analysis using linear regression method with step wise regression technique. From the results of well data crossplot analysis for sand-shale sensitivity analysis and inversion analysis on sandstones filled with hydrocarbon fluid obtained the acoustic impedance value is between 12,000 ft / s * g / cc - 27,000 ft / s*g / cc. In the Middle part around the NN-1, NN-3 and NN4 wells and the Southeastern part of the research area are prospect areas that have gamma ray values with a range of 5-70 API, density with a range of 2.1-2.3 gr / cc and porosity with a range of 18-23% and SW of 10-13% indicating that the hydrocarbon gas accumulated in the research field. The results of the multi-attributes and acoustic impedance show that the sandstones in this field are thight sandstones.

Mapping the Distribution and Characterization of Sandstone Reservoir Using Simultaneous Inversion Method in “OA” Field at Northern Bonaparte Basin

The sandstone reservoir in the “OA” Field is situated in the Northern Bonaparte Basin, which is gas-saturated sandstone. Mineral diagenesis is also causing several reservoir zone areas to become tight sand. The inversion method using acoustic impedance (AI) is less sensitive in distinguishing sandstone and clay rock lithology since they have almost the same impedance. Simultaneous Inversion overcomes this problem by simultaneously inversing partial angle data (near, mid, far) to obtain physical parameters besides acoustic impedance which are expected to be more sensitive in distinguishing lithology and predicting the presence of gas fluids such as shear impedance (SI) and density. These three parameters can be derived as Lame (LMR) parameters. Cross plot analysis shows sensitive physical parameters to predict the distribution of lithology and the presence of gas fluid. Density sensitive in distinguishing lithology which is then inverted, obtaining sandstone cutoff values are 2.3-2.5 (g/cc), tight sand with cutoff 2.5-2.625 (g/cc) and clay stones with cutoff 2.625-2.8 (g/cc). The presence of gas fluid is predicted by inversing the parameter Vp/Vs which has a cutoff of >1.6 and Lambda-rho cutoff of >25 (Gpa)*(g/cc). Analysis of the parameter distribution map shows the distribution of sandstones and the presence of dominant gas fluids in the northern area of the research zone with relatively clean sandstone, compared to relatively more southern regions.

A physical modelling study of the reservoir scale effect on amplitude variation with incidence angle

Abstract The 3D pre-stack seismic data from a physical modeling experiment were employed to investigate the effect of reservoir scales on AVO (amplitude variation with offset or incidence angle). Eight cuboid samples simulating cavernous reservoirs with different widths and the same thickness and elastic parameters were set within a 3D model. 3D seismic data acquisition and processing were conducted. To get the AVO responses of the samples with different widths, trough amplitudes corresponding to the sample tops at different incidence angles were extracted from the pre-stack angle gathers. Amplitude calibration for transducer radiation patterns was conducted on the extracted amplitudes at different angles. AVO analysis was conducted to quantitatively demonstrate the effect of sample scales on AVO characteristics. The effect of sample width was weak when the width was less than 60 m, which was 3/5 of the wavelength. When the width was larger than 60 m, both AVO intercept and gradient gradually increased with the sample width. The AVO gradient peaked at 150 m, which was 1.5 times the wavelength. Cross-plot analysis of AVO intercept and gradient showed the samples were aligned in a straight line when the sample width was less than twice the seismic wavelength. The result in this study partially verified the conclusions of reservoir scale effect on AVO responses drawn from previous numerical modelling studies. For a heterogeneous rectangular reservoir, the effect of reservoir scales on AVO responses could potentially be used to quantitatively estimate reservoir scale.

Integrating seismic acoustic impedance inversion and attributes for reservoir analysis over \u2018DJ\u2019 Field, Niger Delta

Structural interpretation and inversion analysis were used to characterize hard-to-image reservoirs, to predict subsurface interwell reservoir properties for optimum reservoir heterogeneity description, and to fine-tune drilling locations in ‘DJ’ Field, Niger Delta. Post-stacked 3D seismic data, composite well logs, and velocity checkshot data were used for the reservoir analysis. The study entailed mapping of structural framework, horizon picking, wavelet extraction, log editing and correlation, building of low-frequency model, acoustic impedance inversion and crossplot analysis of reservoir properties. Four major antithetic, three regional, and five minor faults were identified. The inversion results revealed an acoustic impedance range of 9700–25,000 ft/s g/cc and porosity range of 25–45% within the hydrocarbon-bearing sands. Crossplot analysis of Poisson and Vp/Vs ratio against acoustic impedance revealed Poisson ratio range of 0.30–0.45 and Vp/Vs ratio range of 1.3–2.50 within the delineated hydrocarbon-bearing sandstone interval. The overall correlation coefficient between the inverted and actual impedance was about 98% across the eight wells. Acoustic impedance slice at 2300 ms revealed low acoustic impedance sand within the range of 13,000–24,000 ft/s g/cc at the western and central part of the field. Comparing the acoustic impedance slice and seismic attribute maps at the target reservoir zones revealed high reflection amplitudes (bright Spots), indicative of hydrocarbon accumulation. The study predicts new and reliable drillable locations, by lithologic/fluid discrimination in the analysis of delineated reservoirs in the study area.

System identification based proxy modeling of a reservoir under iWAG

The high computational cost associated with first principle based numerical reservoir models create a barrier for realizing their full potential. Currently, high fidelity proxy models, capable of mimicking the numerical reservoir models, are used to undertake some of the tasks. Nonetheless, large number of experimental simulations are required to generate modeling data. Moreover, experimental realizations are obtained by sampling static geological properties, hence, the resulting proxy model does not honor the dynamic nature of reservoirs, especially during immiscible water alternating gas injection (iWAG) process. In this study, a system identification based proxy modeling algorithm is developed for a reservoir under iWAG. A case study is used to evaluate the algorithm’s performance in reproducing oil, gas and water production rates. By systematically designing the input pattern, only a single experimental run is used to generate modeling data. A blind validation and a cross plot analysis resulted in an average percentage fit of 89% and a tolerance level of up to 10%, respectively. The algorithm can have potential application for optimization of injection scenario and hence for automatic well control during iWAG.

Cross plot Analysis of Rock Properties from Well Log Data for gas detection in Soku Field, Coastal Swamp Depobelt, Niger Delta Basin

The cross plotting of rock properties for fluid and lithology discrimination was carried out in a Niger Delta oil field using well data X-26 from a given oil field in the coastal swamp depobelt. The data used for the analysis consisted of suites of logs, including gamma ray, resistivity, sonic and density logs only. The reservoir of interest Horizon 1, was identified using the available suite of logs on the interval where we have low gamma ray, high resistivity and low acoustic impedance specifically at depths 10,424ft (3177.24m) to 10 724ft (3268m). We first obtained other rock attributes from the available logs before cross plotting. The inverse of the interval transit times of the sonic logs were used to generate the compressional velocities and the S-wave data was generated from Castagna´s relation. Employing rock physics algorithm on Hampson Russell software (HRS), rock attributes including Vp/Vs ratio, Lambda-Rho and Mu-Rho were also extracted from the well data. Cross plotting was carried out and Lambda Rho (λρ) versus MuRho (μρ) crossplots proved to be more robust for lithology identification than Vp versus Vs crossplots, while λρ Versus Poisson impedance was more robust than Vp/Vs versus Acoustic impedance for fluid discrimination, as well as identification of gas sands. The crossplots were consistent with Rock Physics Templates (RPTs). This implies the possibility of further using the technique on data points of inverted sections of various AVO attributes within the field in areas not penetrated by wells within the area covered by the seismic.

Identification of sedimentary-diagenetic facies and implications for reservoir quality: evidence from the Eocene coarse-grained deposits in the Dongying Depression, Bohai Bay Basin, China

Facies identification is a fundamental issue on evaluation and prediction for siliciclastic sediments. The purpose of this paper is to identify sedimentary-diagenetic facies and investigate the influence of sedimentary-diagenetic facies on reservoir quality in coarse-grained deposits from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin. Based on detailed core observation, thin section petrography combined with conventional wireline logs and core measurement analysis, 12 types of sedimentary- diagenetic facies were divided according to their lithofacies, diagenetic products and related contributions to reservoir quality. The identification and spatial distribution of these sedimentary-diagenetic facies can be determined based on the Bayes discriminant analysis and related cross-plot analysis. The variations of porosity and permeability with burial depth were precisely determined with ±2% for porosity and ±1 × 10–3 μm2 for permeability. The spatial distribution of reservoir porosity and permeability can be predicted quantitatively: high zones of porosity and permeability mainly distributed in the central sections of thick-bedded sandbody, ranging from 6% to greater than 12% for reservoir porosity and from 1 × 10–3 μm2 to greater than 6 × 10–3 μm2; relatively low zones of porosity (less than 6%) and permeability (less than 1 × 10–3 μm2) mainly occur in the bottom and the margin of the thick-bedded sandbody.

INTERPRETASI AKUSTIK IMPEDANSI (AI) MENGGUNAKAN DATA SEISMIK DAN DATA SUMUR UNTUK MENENTUKAN ZONA PROSPEK HIDROKARBON

Research has been conducted to determine the prospect of hydrocarbon zones using the interpretation of acoustic impedance (AI) seismic method in Field X, South Sumatra Basin, Jambi Province. The purpose of this study was to determine the AI value on the distribution map of AI values for hydrocarbon prospect zones in Field X. In this study the data used were 3D seismic data with PSTM (Post-Stack Time Migration) type, and well data. Data processing using Hampson and Russell (HRS) software used in the mining and petroleum fields has a function for subsurface modeling below the ground surface of reservoir characterization. Well data will be linked to seismic data so that the well data will be in the actual position. The distribution of hydrocarbon prospect zones in PEV-1 well can be seen first in crossplot analysis at a depth of 1760-1798 m with AI cutoff value indicated as sandstone ranging from 8450 (m s)*(g/cc) and above, with a high correlation value 0.818 time shift 0 ms. Picking horizon is carried out to determine the target zone layer and its continuity laterally on seismic volume so that a model based can be done as an initial subsurface description below the soil surface at PEV-1 well. Then the next step is a post-stack analysis based model to find out how much the error value of the target zone prediction with certain parameters through the PEV-1 well data. So with a correlation value of 0.936429 and an error value of 0.35227 in the post-stack analysis model based, AI inversion in the PEV-1 well layer which is the target zone of the hydrocarbon prospect is indicated by the range of 8450 (m/s)*(g/cc) which is indicated as sandstone.

A Modified Approach for Volumetric Evaluation of Shaly Sand Formations from Conventional Well Logs: A Case Study from the Talhar Shale, Pakistan

In potential shale-gas basins, due to limited data availability, volumetric evaluation of shaly sand formations is a challenging task for researchers, as well as for exploration and production companies, with the use of conventional well logs. This paper is intended to provide a better understanding of volumetric evaluation in unconventional shaly sand formations. Petrophysical parameters derived from conventional well logs play a significant role in the volumetric estimation of shale. The assessment was performed using numerical equations, well log indexes, and cross-plot analysis. Deep and shallow resistivity logs overlap with each other; this phenomenon indicates that the Talhar Shale in Pakistan can be considered to be an unconventional formation. The average values of shale volume, total porosity, effective porosity, and matrix volume are 29, 16, 12, and 59%, respectively. The Talhar Shale is dominantly characterized by intergranular porosity. It has been observed that at low shale volumes (10–20%), porosities (total and effective), and resistivity measurements (deep, shallow, and microlaterolog), well log signatures have close contents with the shale volume. In contrast, for the remainder of the shale volume, there is significant separation of log signatures. Well log indexes indicate significant responses in 10–20% shale volume zones.

Characterization of hydrocarbon reservoir by pore fluid and lithology using elastic parameters in an x field, Niger delta, Nigeria

Quantitative rock physics analysis was carried out to determine the lithology and pore fluid of a reservoir in the Niger Delta. Density, compressional wave velocity and shear wave velocity logs were used as input to calculate elastic parameters such as velocity ratio, Poisson’s ratio, and Bulk Modulus, after estimating the hydrocarbon reservoir in the X field. The calculated velocity ratio log was used to differentiate between sand, sandstone and shale. Poisson’s ratio and velocity ratio were used delineate pore fluid content; gas sand, oil sand and sandstone formation from cross plot analysis. The reservoir in the field lies ranges from 9050 - 9426.5ft, (2760.25 – 2874.93m), this confirm what is obtained in the Niger Delta Basin. The Net Pay zones show an economical viable reservoir, it Net pay depth is 39 – 73.5ft. The Porosity and Permeability of the reservoirs suggested a productivity hydrocarbon reservoir. The reservoir lies between Gas sands, Oil sands and Brine sands, reservoir 2 and reservoir 3 are oil sand reservoirs while reservoir 1 lies between an oil sand and a brine sand.

Discrimination of pore fluid and lithology of a well in X Field, Niger Delta, Nigeria

Quantitative rock physics analyses were used to determine the lithology and pore fluid of a reservoir in the Niger Delta. Inaccurate prediction of lithology and pore fluid results in the inaccurate determination of other petrophysical properties and parameters such as porosity, permeability, and net pay. This research is to predict lithology and pore fluid using rock physics analysis. However, reservoir zones were also predicted. Density, compressional wave velocity, and shear wave velocity logs were used as input to calculate elastic parameters such as velocity ratio, Poisson’s ratio, and bulk modulus. The calculated velocity ratio log was used to differentiate between sand and shale. Poisson’s ratio and velocity ratio using Goodway interpretation template were carried out and used to delineate pore fluid content, gas sand, oil sand, and sandstone formation from crossplot analysis.

Petrophysical evaluation and lithology delineation using cross-plots analysis from some onshore wells in the Nigerian-delta, west Africa

Reservoir rock attributes such as porosity, permeability, pore-size geometry and net-to-gross ratio can grossly be affected by inaccurate delineation of the sand intervals. It is therefore pertinent that well log cross-plot is utilized to accurately delineate the sand body and correctly evaluate the petrophysical properties of the mapped sandstone intervals. Three wells (A, B and C) were studied from which analysis of various cross-plots were done. The cross-plots of the density and gamma ray, Acoustic impedance and VpVs ratio with various colour indicators such vertical depth, porosity, resistivity, water saturation etc. were generated using the Hampson Russel software. The hydrocarbon interval in the area occurs between 5870ft to 8900ft for well-A, 5500ft to 5910ft for well-B and 5700ft to 7230ft for well-C as interpreted from the well logs with an average porosity range from 26 to 39%. Cross-plot analysis was carried out to validate the sensitivity of the rock attributes to reservoir saturation condition. The cross-plot results clusters shows two major lithologies of sandstone and shale with occasional intercalation of sand and shale units. For the three wells considered, ten reservoirs were observed. Fluid detection analysis shows that reservoirs A1-A2 (well-A), B1-B2 (well-B), C3-C4 (well-C) were found to contain oil, while reservoir A3-A4 (well-A) and C1-C2 (well-C) contains gas. This study has shown that the cross-plots approach can be used to accurately delineate reservoirs for further formation evaluation. It therefore means that an outright estimation of petrophysical properties on wrongly delineated reservoirs can significantly affect the porosity, permeability, pore-size geometry and net-to-gross ratio of the reservoir units.

Search for Trends and Periodicities in Inter-hemispheric Sea Surface Temperature Difference

Understanding the role of coupled solar and internal ocean dynamics on hemispheric climate variability is critical to climate modelling. We have analysed here 165 year long annual northern hemispheric (NH) and southern hemispheric (SH) sea surface temperature (SST) data employing spectral and statistical techniques to identify the imprints of solar and ocean–atmospheric processes, if any. We reconstructed the eigen modes of NH-SST and SH-SST to reveal non-linear oscillations superimposed on the monotonic trend. Our analysis reveals that the first eigen mode of NH-SST and SH-SST representing long-term trend of SST variability accounts for ~ 15–23% variance. Interestingly, these components are matching with first eigen mode (99% variance) of the total solar irradiance (TSI) suggesting possible impact of solar activity on long-term SST variation. Furthermore, spectral analysis of SSA reconstructed signal revealed statistically significant periodicities of ~ 63 ± 5, 22 ± 2, 10 ± 1, 7.6, 6.3, 5.2, 4.7, and 4.2 years in both NH-SST and SH-SST data. The major harmonics centred at ~ 63 ± 5, 22 ± 2, and 10 ± 1 years are similar to solar periodicities and hence may represent solar forcing, while the components peaking at around 7.6, 6.3, 5.2, 4.7, and 4.2 years apparently falls in the frequency bands of El-Nino-Southern Oscillations linked to the oceanic internal processes. Our analyses also suggest evidence for the amplitude modulation of ~ 9–11 and ~ 21–22 year solar cycles, respectively, by 104 and 163 years in northern and southern hemispheric SST data. The absence of the above periodic oscillations in CO2 fails to suggest its role on observed inter-hemispheric SST difference. The cross-plot analysis also revealed strong influence of solar activity on linear trend of NH- and SH-SST in addition to small contribution from CO2. Our study concludes that (1) the long-term trends in northern and southern hemispheric SST variability show considerable synchronicity with cyclic warming and cooling phases and (2) the difference in cyclic forcing and non-linear modulations stemming from solar variability as a possible source of hemispheric SST differences.

Reducing Uncertainties in Hydrocarbon Prediction through Application of Elastic Domain

The application of lithology and fluid indicators has helped the geophysicists to discriminate reservoirs to non-reservoirs from a field. This analysis is conducted to select the most suitable lithology and fluid indicator for the Malaysian basins that could lead to better eliminate pitfalls of amplitude. This paper uses different rock physics analysis such as elastic impedance, Lambda-Mu-Rho, and SQp-SQs attribute. Litho-elastic impedance log is generated by correlating the gamma ray log with extended elastic impedance log. The same application is used for fluid-elastic impedance by correlation of EEI log with water saturation or resistivity. The work is done on several well logging data collected from different fields in Malay basin and its neighbouring basin. There's an excellent separation between hydrocarbon sand and background shale for Well-1 from different cross-plot analysis. Meanwhile, the Well-2 shows good separation in LMR plot. The similar method is done on the Well-3 shows fair separation of silty sand and gas sand using SQp-SQs attribute which can be correlated with well log. Based on the point distribution histogram plot, different lithology and fluid can be separated clearly. Simultaneous seismic inversion results in acoustic impedance, Vp/Vs, SQp, and SQs volumes. There are many attributes available in the industry used to separate the lithology and fluid, however some of the methods are not suitable for the application to the basins in Malaysia.

The organic petrology of graptolites and maturity assessment of the Wufeng–Longmaxi Formations from Chongqing, China: Insights from reflectance cross-plot analysis

The Wufeng–Longmaxi Formations (Ordovician-Silurian) contain abundant graptolite-bearing shales, which have proven to be one of the most successful shale gas exploration targets in China and can provide an opportunity to study the organic petrology of graptolites. Organic petrology analysis has been conducted on these organic-rich shales in order to depict the shape of graptolite reflectance indicating surface (GRIS) and their organic maturity. Granular and non-granular graptolites have been observed in the studied sediments, and non-granular graptolites were the major graptolite species. The strong positive correlation between the maximum reflectance (Rmax) and bireflectance of graptolites indicates that the graptolites display stronger anisotropy with maturation. A linear coalification path of graptolite fragments was observed in the studied sediments. A remarkable decrease in the minimum reflectance (Rmin) values of graptolites was observed when Rmax exceeds 6%. The samples with lower thermal maturity (Rmax≤2%) tend to be biaxial neutral, whereas higher rank samples display biaxial negative characteristics. The GRIS shape becomes more oblate with maturation. The Wufeng–Longmaxi shales from the southeastern Chongqing and Wuxi areas of the Chongqing municipality, China, are post mature and in the dry gas zone, whereas samples from the Chengkou area of the Chongqing municipality, China, are still in the oil window. Their different burial and thermal histories controlled their thermal maturity.

Total and speciation analyses of heavy metals in the sand fraction of Nigerian oil sands for human and ecological risk assessment

ABSTRACTTotal metal analysis and metal speciation of sand fraction of Nigerian oil sands were done to extract and partition heavy metals into six operationally defined fractions in order to assess environmental and health implications of the oil sand development. Soxhlet extraction of bitumen from the oil sand was done using toluene. Traces of water and extracting solvent were removed at 70°C from the sand fraction using oven. Elemental analysis was performed by Atomic Absorption Spectrophotometry. The heavy metals except Cu have low Pollution index values. Negative geo-accumulation index was obtained for the metals except Cu, indicating that the oil sands were uncontaminated with the metals. Strong and significant positive correlations existed between Fe/Pb, Zn/Cu, Cd/Pb, Ni/Cd, Ni/Pb, Cd/Fe, Ni/Fe and Cr/Zn, while strong and significant negative correlations existed between Mn/Pb and As/Mn, indicating common sources or chemical similarities and vice versa. T-test results indicated significant differences between the concentrations of the metals. Cross-plot analysis showed strong positive correlation between the sand fraction and Nigerian bitumen. Speciation analysis indicated highest and lowest indices of metal mobility for Zn and Pb, respectively. This study concluded that the sand fraction may not pose any environmental risks from elemental point of view.

Chemical analysis of resin fraction of Nigerian bitumen for organic and trace metal compositions

ABSTRACTStudies of organics and trace metals of the resin fraction of Nigerian bitumen were carried out with an opinion to establish the environmental consequences of the fossil fuel and as well as characterize them. Bitumen samples were extracted from oil sands via Soxhlet extraction procedure using toluene as the extracting solvent. The bitumen samples were deasphaltened to obtain maltene by dissolving the samples in n-pentane (1:40). Column chromatography was used to fractionate the maltene into its components (saturates, aromatics, and resin) using silica gel as adsorbent and dissimilar solvents of varying polarity (n-hexane for saturates, toluene for aromatics, and methanol for resins). The organic compounds of the resin fraction were examined using Fourier Transform Infrared Spectrophometry (FTIR) and the elemental concentrations were determined using Inductively Coupled Plasma Optical Emission Spectrophotometry (ICP-OES). The infrared spectra of all the resin samples presented distinctive bands including expected polar functional groups which confirmed that the resin was cleanly eluted from the bitumen samples. The concentrations of the analyzed trace metals in the resin fraction were found to be high compared to other fractions and this was confirmed by their T-test values. Fe had the highest average concentration of 34.81 ± 14.37 mg/kg and a range of 19.35–47.48 mg/kg, while Mn had the least mean concentration of 0.94 ± 0.71 mg/kg and a range of 0.35–1.88 mg/kg. The result of cross-plot analysis revealed that resin fraction has strong and positive inter elemental correlation with the NSO and asphaltene fraction of the same bitumen samples. The overall results indicated that processing of the bitumen and/or its resin may cause catalyst poisoning and fouling, corrosion of equipment and pipelines, and particulate emissions into the environment and therefore adequate demetalisation of the fossil fuel must be done before processing.

Rock physics modeling in sand reservoir through well log analysis, Krishna-Godavari basin, India

Rock physics modeling of sandstone reservoir from gas fields of Krishna-Godavari basin represents the link between reservoir parameters and seismic properties. The rock physics diagnostic models such as contact cement, constant cement and friable sand are chosen to characterize reservoir sands of two wells in this basin. Cementation is affected by the grain sorting and cement coating on the surface of the grain. The models show that the reservoir sands in two wells under examination have varying cementation from 2 to more than 6%. Distinct and separate velocity-porosity and elastic moduli-porosity trends are observed for reservoir zones of two wells. A methodology is adopted for generation of Rock Physics Template (RPT) based on fluid replacement modeling for Raghavapuram Shale and Gollapalli Sandstones of Early Cretaceous. The ratio of P-wave velocity to S-wave velocity (Vp/Vs) and P-impedance template, generated for this above formations is able to detect shale, brine sand and gas sand with varying water saturation and porosity from wells in the Endamuru and Suryaraopeta gas fields having same shallow marine depositional characters. This RPT predicted detection of water and gas sands are matched well with conventional neutron-density cross plot analysis.

Lithology and mineralogy recognition from geochemical logging tool data using multivariate statistical analysis

The availability of a deep well that penetrates deep into the Ultra High Pressure (UHP) metamorphic rocks is unusual and consequently offers a unique chance to study the metamorphic rocks. One such borehole is located in the southern part of Donghai County in the Sulu UHP metamorphic belt of Eastern China, from the Chinese Continental Scientific Drilling Main hole. This study reports the results obtained from the analysis of oxide log data. A geochemical logging tool provides in situ, gamma ray spectroscopy measurements of major and trace elements in the borehole. Dry weight percent oxide concentration logs obtained for this study were SiO2, K2O, TiO2, H2O, CO2, Na2O, Fe2O3, FeO, CaO, MnO, MgO, P2O5 and Al2O3. Cross plot and Principal Component Analysis methods were applied for lithology characterization and mineralogy description respectively. Cross plot analysis allows lithological variations to be characterized. Principal Component Analysis shows that the oxide logs can be summarized by two components related to the feldspar and hydrous minerals. This study has shown that geochemical logging tool data is accurate and adequate to be tremendously useful in UHP metamorphic rocks analysis.

An intelligent approach to brittleness index estimation in gas shale reservoirs: A case study from a western Iranian basin

Brittleness index is one of the key parameters for characterization of unconventional shale gas reservoirs and screening favorable hydraulic fracturing candidates. Brittleness index in shale reservoirs are commonly calculated based on their mineralogical composition or alternatively from dynamic Young's modulus and Poisson's ratio profiles. However, shortage of mineralogical and shear slowness logging data, mainly due to their high costs, may restrict application of these methods in many wells. To overcome this limitation, this paper establishes an intelligent approach to evaluate brittleness from conventional well logs based on data collected from to potential shale gas formations in one of Iranian Basins. The mineralogical analysis (XRD) revealed that both shales are mainly composed of quartz and carbonate minerals and they are highly brittle. Afterward, a rigorous approach, namely adaptive neuro-fuzzy inference system (ANFIS) was employed to develop a robust correlation of brittleness index with conventional well logs. Additionally, a comprehensive statistical and graphical investigation including cross plot analysis, sensitivity analysis by employing a relevancy factor and Leverage approach for outlier data detection was conducted. The results well prooved that the proposed ANFIS model is statistically valid and reliable. Moreover, the developed model were compared to the pre-existing empirical models, yielding the supremacy and superiority of the proposed model over the pre-existing empirical correlations. The model was finally applied to predict brittleness index in the gas shale formations along a well in the study area. Finally, based on estimated brittleness, the potential depth intervals were identified for creation of complex fracture network within gas shale formations.