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

Abstract

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. KEY WORDS: ultrahigh pressure metamorphic rock, CCSD main hole, well logging response.

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

“…Metal mineral content of ultramafic rock (Fe 2 O 3 is 5.81%-8.12%; FeO is 2.8%-6.11%) of the CCSD-MH is a little higher than those of the other rocks Zhang et al, 2006b), so its resistivity is relatively low. The neuron log shows that the hydrogen content of ultramafic rock of CCSD-MH is high (Luo and Pan, 2010), because the ultramafic rock is serpentinized peridotite . Therefore, the serpentinized effect may cause the increase of metal mineral and hydrogen content and the decrease of resistivity.…”

“…Niu et al (2004) briefly introduced the well logging methods performed at the CCSD-MH (100-2 000 m) and some preliminary studies they have finished. The well logs of the entire borehole were displayed and analyzed in several references (Luo and Pan, 2010;Luo et al, 2008).…”

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

“…These occur in a range of continental and oceanic settings, where a number of boreholes have been and continued to be drilled in order to provide information of the Earth's crust by means of geophysical surveys and geological mapping (Harvey et al, 2005). Several studies have been reported by geophysicists based on crystalline rocks using well logging data (see for example Pratsone et al, 1992;Pechnig et al,1997Pechnig et al, , 2005Bartetzko et al, 2005;Maiti and Tiwari, 2009;Luo and Pan, 2010;. The general understanding collected from these studies show that, the well logging data from crystalline rocks are not simple to analyze because of their complicated geological characteristics and the difficulty in understanding and using the intensive information content in these data.…”

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

“…Gu et al (2009) constructed the lithology recognition model combining the logging response and several well logs of different rocks with the method of cluster analysis and stepwise discriminant analysis. Luo and Pan (2010) used core-log correlation and cross-plotting methods, and the results allowed the authors to conclude that the lithology is mainly comprised orthogneiss, paragneiss, eclogite, amphibolite, and ultramafic rocks. Bosch et al (2013) used fuzzy logic for lithology prediction from well log data of the German Continental Deep Drilling Program (KTB).…”