Continental Scientific Drilling Main Hole Research Articles

Analysis of situ elemental concentration log data for lithology and mineralogy exploration— A case study

Metamorphic rocks are diverse with more compositions, structures, and textures that are complex. Rock type identification and prediction from metamorphic rocks using well log data are difficult tasks. This study shows the use of cross plot technique, Pearson correlation, and factor analysis in metamorphic rocks interpretation using borehole geochemical data from the 4390–5089 m interval depth of the Chinese Continental Scientific Drilling Main hole. Lithological identification abilities, correlation between geochemical and geophysical logs, and build a factor model which link in situ chemical element to minerals were studied. The results show that Potassium and Thorium logs are the most discriminating logs in metamorphic rocks. Pearson correlation shows that Potassium and Thorium are the largest contributors to the gamma ray responses. Factor analysis results show a 2 factor model-where factor 1 (amphibole mineral) and factor 2 (K-feldspar mineral) described 76.261% of the variation in log responses. These statistical methods can be a very helpful tool in helping the task of geoscientists in the context of research drillings.

Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.

Application of well logs integration and wavelet transform to improve fracture zones detection in metamorphic rocks

The traditional approach to fracture zones detection is based on a visual inspection of core samples, or the application of special well logging program represented by imaging well logging. However, not all wells are fully cored, as the core description of fractures in laboratory is time consuming and expensive. Additionally, the interpretation of special well logging program in detecting fracture zones also needs the geologists' experience. This study provides a complete set of methodology for fracture zones detection and fracture numbers calculation in which wavelet analysis is utilized. For the sensitive well logs in fracture zones, we have made a contrast between well logs and imaging logging that shows up density, caliper, resistivity and acoustic logs are more or less suitable. To meet little changes with the same trends in one curve which indicates the present of fracture zones, we proposed a method that integrates the above four well logs into one, named fractured integrated index. Decomposing the integrated curve with discrete wavelet analysis shows that the detailed wavelet coefficients (cD1) energy are highly suitable for detection of fracture zones. In addition to the above four well logs, microsphere focusing logging is also used to enhance the accuracy of the detecting analysis. The method is applied to Chinese Continental Scientific Drilling Main Hole located about 17 km southwest of Donghai in the southern segment of the Sulu UHP terrane, and the results are promising in accordance with the imaging logging and cores information.

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.

Performance of the synergetic wavelet transform and modified K-means clustering in lithology classification using nuclear log

Accurate lithology identification is fundamentally crucial to reservoir evaluation from geophysical well logs. However, the traditional way of lithological identification is carried out in laboratory, which is not only expensive, but also time consuming in its interpretation. In this study, the synergetic wavelet transform and modified K-means clustering techniques are performed to classify metamorphic rocks from Chinese Continental Scientific Drilling Main Hole (CCSD-MH). At the beginning, different wavelet functions in different well logs are presented to detect lithologic interfaces. Meanwhile, the Haar wavelet and GR are determined to be the optimum wavelet function and well log, and the range of the optimum scales is about 8–15m in the reference well. After that, a fast and practical K-means clustering algorithm is employed to make a classification of stratigraphy into 5 groups, which are demarcated from the performance of wavelet transform. The results achieved are in accordance with the stratigraphic column and have a higher accuracy compared to the previous studies, indicating that the combination of the wavelet transform and modified K-means clustering can improve the accurate rate for the classification of metamorphic rocks in CCSD-MH.

High-quality heat flow determination from the crystalline basement of the south-east margin of North China Craton

Very few of heat flow data have come from the crystalline basement in the North China Craton but rather from boreholes in the sedimentary cover of oil–gas basins. Explorations for hot dry rock (HDR) geothermal resources and porphyry gold deposits in eastern China offer now valuable opportunities to study the terrestrial heat flow in the crystalline basement. In this study, we obtained continuous temperature logs from two boreholes (the LZ borehole with a depth of 3471m and the DR borehole with a depth of 2179m) located in the south-east margin of the North China Craton. The boreholes have experienced long shut-in times (442days and 261days for the LZ borehole and DR borehole, respectively); thus, it can be expected that the temperature conditions have re-equilibrated after drilling and drill-mud circulation. Rock thermal conductivity and radiogenic heat production were measured for 68 crystalline rock samples from these two boreholes. The measured heat-flow density was determined to be 71.8±2.3mWm−2 (for the LZ borehole) and 91.5±1.2mWm−2 (for the DR borehole). The heat flow for the LZ borehole is close to the value of 75mWm−2 determined in the Chinese Continental Scientific Drilling main hole (CCSD MH), both being in the Sulu-Dabie orogenic belt and thus able to verify each other. The value for the DR borehole is higher than the above two values, which supports former high heat-flow values determined in the Bohai Bay Basin.

Application of dimensionality reduction technique to improve geophysical log data classification performance in crystalline rocks

Abstract In this study, dimensionality reduction technique to improve geophysical log data classification performance in crystalline rocks is presented. In fact, in complex geological situations such as the study area in context, more complex nonlinear functional behaviors exist for well log classification purpose; thus posing challenges in accurate identification of log curves for this purpose. Dimensionality reduction (DR) using Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA) were used here to reduce the dimensionality of the original log set of Chinese Continental Scientific Drilling Main Hole to a convenient size, and then feed the reduced-log set into the classifiers. Three classifiers were addressed, namely, Support vector Machines, Feed forward Back Propagation and Radial Basis Function Neural Networks in the classification of metamorphic rocks. The strategy of combining dimensionality reduction methods and classifiers was demonstrated and discussed. The results showed that the reduced log sets found from DR can separate the metamorphic rocks types better or almost as well as the original log set. Therefore LDA and PCA can be suitable to be performed before geophysical well log data classification in the context of crystalline rocks.

The classification in metamorphic rocks using modified fuzzy cluster analysis from geophysical log data: evidence from Chinese Continental Scientific Drilling Main Hole

Lithology is one of the most important data in evaluating reservoir, and is mainly carried out by cores recovery in laboratory which is very expensive, and its interpretation is time consuming. Accurate identification of lithology is fundamentally crucial to evaluate reservoir from geophysical log data. Pattern recognition and statistical analysis have been proved to be the most powerful methods for constructing optimal model in lithology recognition. To address this issue, a fast and practical K-means clustering algorithm is proposed in order to better deal with lithology recognition from geophysical log data. Based on the traditional K-means clustering algorithm, Euclidean distance is replaced by Mahalanobis distance; the initial cluster centers are acquired from the average of characteristic values but not selected randomly, in addition, adding weight value in each characteristic value of the objective function, and thus a lithology recognition model named modified K-means clustering is established. The method is applied to identify the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) metamorphic rocks. Compared with the traditional K-means clustering, the accurate rate of the modified K-means clustering in lithologic identification has improved for the same 45 samples, raised 11.11 %. According to the modified K-means cluster algorithm, nine kinds of lithology cluster centers are acquired from 45 samples. The classes of the samples can be determined by analyzing the hamming approach degree curves, which is calculated by the undetermined samples and 9 cluster centers. The predicted results and the core recovery are exactly the same by comparison. The hamming approach degree can identify the whole well of CCSD-MH lithology effectively and accurately. This model may be made applications to other areas.

Capability of self-organizing map neural network in geophysical log data classification: Case study from the CCSD-MH

Well log interpretation is one of the prime sources of information for deep lithology in drilling research. Because of the complex geological features of the crystalline metamorphic rocks, more complex nonlinear functional behaviors exist for well log interpretation purposes. Hence, establishing a prediction technology that can accurately interpret/classify well log data in terms of lithology is of major significance. This study, for the first time, explores the application of self-organizing map neural network (SOM) in the classification of metamorphic rocks from Chinese Continental Scientific Drilling Main Hole (CCSD-MH) log data. For this purpose, a total of 33,326 data points derived from resistivity, P-wave velocity, bulk density, photoelectric absorption capture cross section, gamma ray, potassium content and neutron logs were used as an input pattern to a SOM to classify lithology in five categories: orthogneiss, paragneiss, eclogite, amphibolite and ultramafic rocks. Comparison of SOM results to those of feed-forward neural network (FFNN) was also carried out. The cross-validation method was used to investigate the robustness of the two neural networks in terms of classification accuracy in the context of lithology clustering tasks by sampling rotation. Statistical tests such as student paired samples t-test was carried out to guide in classification decision of the CCSD-MH data. The results of this study have proven that SOM appears to be comparable to FFNN in classifying lithology using geophysical log data from crystalline rocks. This proposed SOM approach can serve as practical alternative technology to be used in drilling research.

Prediction of porosity in crystalline rocks using artificial neural networks: An example from the Chinese Continental Scientific Drilling Main hole

Porosity plays an important part of understanding permeability and fluid flow within the continental, crystalline rocks. Geophysical well logs are presently the most consistent means of providing continuous information for porosity estimation. However, it is difficult to interpret geophysical well logs data in crystalline rocks due to their complex geological features and the difficulty in understanding and using the complex and intensive information content in these data. Motived by the successful prediction abilities of artificial neural networks (ANN) to solve different problems in geophysics, this study explore the applicability of using ANNs to predict porosity in continental, crystalline rocks. This ANN technique is calibrated on Chinese Continental Scientific Drilling Main Hole (CCSD-MH) data, which provides core porosity data combined with four geophysical well logs (density, neutron porosity, sonic and resistivity). The data from CCSD-MH is utilized to train feed-forward backpropagation (FFBP) neural network and radial basis function (RBF) neural network to derive a relationship between geophysical well logs and porosity, and hence predict porosity accurately. The findings demonstrate that ANNs provide better performances with sets of three geophysical well logs (density, sonic and resistivity) than regression technique. Comparison of FFBP to RBF showed that RBF reveals better stability and more accurate performances than FFBP. Based on the success achieved in this study, this intelligence artificial technique can be a very advantageous tool in facilitating the task of geophysicists in the framework of research drillings in continental crust.

Resistivity logs of the Chinese Continental Scientific Drilling main hole: Implication for the crustal electrical structure of Dabie-Sulu Terrane, Central-Eastern China

To understand the electrical structure of the Sulu (苏鲁) terrane, we analyzed the resistivity logs of the Chinese Continental Scientific Drilling main hole (CCSD-MH) and obtained the statistical characteristics of the resistivity log of ultrahigh-pressure (UHP) metamorphic rocks. To check the logs’ quality, we compared the resistivity log data with the core test data. The two datasets show a good coherence. On the whole, the resistivity of the UHP metamorphic rocks from the CCSD-MH is very high (mostly higher than 1 000 Ω·m). From orthogneiss to paragneiss, amphibolite, retrograded eclogite, rutile eclogite, phengite eclogite, and ultramafic rock, the resistivity decreases gradually. The fracture zone, enrichment of metal minerals, and serpentinization of peridotite can probably cause the decrease of resistivity. Comparing the electrical structure of the crust of Dabie (大别) terrane and Sulu terrane, we can conclude that (1) the electrical structure of the crust of Dabie terrane and Sulu terrane shows a very big difference, (2) the high-resistivity zone of the north Sulu orogen extended to a very great depth (60 km), but the high-resistivity zone of Dabie orogen is over the depth of 25 km.

Well logging responses of UHP metamorphic rocks from CCSD main hole in Sulu terrane, eastern central China

The lithologies of the Chinese Continental Scientific Drilling main hole (CCSD-MH) are mainly comprised of orthogneiss, paragneiss, eclogite, amphibolite, and ultramafic rocks. The statistical results of logs of CCSD-MH indicate that ultramafic rocks are characterized by very high CNL (neutron log) and very low GR (gamma ray log) and RD (resistivity log); eclogites are characterized by high DEN (density), V P (P-wave velocity) and PE (photoelectric absorption capture cross section); orthogneiss and paragneiss are characterized by high GR, U (uranium content), Th (thorium content), K (potassium content) and RD, and low DEN, PE, and CNL; logging values of amphibolite are between the logging values of eclogites and paragneiss. In addition, the logs could reflect the degree of retrograde metamorphism of eclogites. The upper section (100–2 000 m) shows higher DEN, PE, V P, and lower GR, U, Th, K, RD than the lower section (2 000–5 000 m). Most logs of the upper section are more fluctuant than those of the lower section. This indicates that the upper section has more heterogeneities than the lower section. The cross plots of logs indicate that DEN, GR, K, and CNL are more powerful in identifying ultrahigh pressure metamorphic (UHPM) rocks at the CCSD-MH. GR value of the rocks from CCSD-MH shows obviously an increasing trend from ultramafic rock (the most mafic rocks at CCSD-MH) to orthogneiss (the most acid rocks at CCSD-MH). On the contrary, DEN value decreases from the ultramafic rock to the orthogneiss. CNL log is a good indicator of the content of structure water in crystalline rocks.

Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating

AbstractThe 5‐km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)‐metamorphic rocks consisting mainly of eclogite, gneiss and garnet‐peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low‐P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite‐facies minerals such as phengite and jadeite. The zircon cores give highly variable 206Pb/238U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO2, Al2O3, K2O, FeO, MgO, Na2O and H2O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.

Petrological and geochronological constraints on the formation, subduction and exhumation of the continental crust in the southern Sulu orogen, eastern-central China

Detailed petrological, geochemical and geochronological studies were carried out for the core samples from the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) with a final depth of 5158 m. This borehole has penetrated into an ultrahigh-pressure (UHP) metamorphic slice consisting mainly of eclogites, gneisses, garnet–pyroxenites and garnet–peridotites. Geochemical characteristics indicate that their protoliths are igneous rocks, and occur in a continental rifting tectonic setting. Quartz-, rutile- and ilmenite-rich eclogites from 0 to 710 m occur as alternating layers; the eclogites, together with interlayers of peridotites and gneisses form a layered ultramafic–mafic–acidic intrusion, which was formed by extensive fractional crystallization of basaltic magma in continental environments. The granitic gneisses from 1190 to 1505 m and 3460 to 5118 m show affinity to within-plate granite, whereas the granitic gneisses from 710 to 1190 m and 1505 to 3460 m exhibit characteristics of volcanic-arc granite. Zircon U–Pb dating demonstrates that the magmatic zircon cores, which have relatively high Th/U ratios (mostly > 0.4), from both eclogites and gneisses, yield the same age at c. 788.8 Ma, suggesting that the protoliths of UHP rocks were formed by bimodal magmatism in Neoproterozoic rifting tectonic zones along the northern margin of the Yangtze Plate, in response to the breakup of the supercontinent Rodinia. U–Pb dating of metamorphic zircons with coesite and other eclogite-facies mineral inclusions and with relatively low Th/U ratios (mostly < 0.14) gives similar Triassic ages, which define two main zircon-forming events at 221.1 Ma and 216.7 Ma. We suggest that the older weighted mean age represents the peak-UHP metamorphic event at a pressure of 5.0 GPa (corresponds to ∼ 165 km depth), whereas the younger mean age reflects the UHP/HP retrograde event at a pressure 2.8 GPa (∼ 92 km depth). Therefore, a maximum rate of vertical movement during early exhumation of the UHP rocks from the Sulu orogen would be 17 mm/year, which is quite similar to initial exhumation rates (16 to 35 mm/year) of many UHP terranes in the world.

Integrated Log Interpretation in the Chinese Continental Scientific Drilling Main Hole (Eastern China): Lithology and Mineralization

Delineation of lithology and mineralization of different rock units of the Chinese Continental Scientific Drilling Main Hole (CCSD-MH), Eastern China, is the main objective of this study. Different logging operations were conducted in the studied area. Subduction and metamorphism lead to high compressional wave velocity (Vp) and low neutron density (CNL) values of different rocks.

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100-2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations. (1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and ~910°C. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910°C and ~37 kbar for rutile eclogite, and ~850°C and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530-560°C and 7-9 kbar. The presence of K-feldspar-albite symplectite on phengite and minor interstitial K-feldspar implies that post-peak decompressional partial melting occurred locally during rapid exhumation of the subducted UHP slab. Two alternative P-T paths may be constructed: (1) the first possesses a different pre-peak stage but similar peak UHP to post-peak decompression cooling for rutile and phengite eclogite; (2) the second shows a consistent evolution from a pre-peak stage via peak UHP stage to a post-peak stage of near-isothermal decompression, followed by near-isobaric cooling for both rutile and phengite eclogite. We prefer the first P-T path, because the inferred paths show a clear increase in pressure and temperature from near-peak to peak UHP stage, and this may be associated with subductionrelated tectonism as a result of continental collision between the North China and Yangtze blocks. Rapid subduction and fast retrograde exhumation is likely responsible for preservation of some pre-peak inclusion assemblages and prograde mineral growth zoning.