Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin

Abstract

BackgroundIntegrated Ocean Drilling Program Expedition 338 was the second scientific expedition with D/V Chikyu during which riser drilling was conducted as part of the Nankai Trough Seismogenic Zone Experiment. Riser drilling enabled sampling and real-time monitoring of drilling mud gas with an onboard scientific drilling mud gas monitoring system (“SciGas”). A second, independent system was provided by Geoservices, a commercial mud logging service. Both systems allowed the determination of (non-) hydrocarbon gas, while the SciGas system also monitored the methane carbon isotope ratio (δ13CCH4). The hydrocarbon gas composition was predominated by methane (> 1%), while ethane and propane were up to two orders of magnitude lower. δ13CCH4 values suggested an onset of thermogenic gas not earlier than 1600 meter below seafloor. This study aims on evaluating the onboard data and subsequent geological interpretations by conducting shorebased analyses of drilling mud gas samples.ResultsDuring shipboard monitoring of drilling mud gas the SciGas and Geoservices systems recorded up to 8.64% and 16.4% methane, respectively. Ethane and propane concentrations reached up to 0.03 and 0.013%, respectively, in the SciGas system, but 0.09% and 0.23% in the Geoservices data. Shorebased analyses of discrete samples by gas chromatography showed a gas composition with ~0.01 to 1.04% methane, 2 – 18 ppmv ethane, and 2 – 4 ppmv propane. Quadruple mass spectrometry yielded similar results for methane (0.04 to 4.98%). With δD values between -171‰ and -164‰, the stable hydrogen isotopic composition of methane showed little downhole variability.ConclusionsAlthough the two independent mud gas monitoring systems and shorebased analysis of discrete gas sample yielded different absolute concentrations they all agree well with respect to downhole variations of hydrocarbon gases. The data point to predominantly biogenic methane sources but suggest some contribution from thermogenic sources at depth, probably due to mixing. In situ thermogenic gas production at depths shallower 2000 mbsf is unlikely based on in situ temperature estimations between 81°C and 85°C and a cumulative time-temperature index of 0.23. In conclusion, the onboard SciGas data acquisition helps to provide a preliminary, qualitative evaluation of the gas composition, the in situ temperature and the possibility of gas migration.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-014-0015-8) contains supplementary material, which is available to authorized users.

Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

“…The C 2 /C 3 ratios are greater than ~1 (table S1), suggesting that C 2+ hydrocarbons are predominantly derived from a thermogenic source in which ethane is generally enriched to propane ( 43 ). Thermogenic gas here was probably generated from sedimentary organic matter below ~2 kmbsf, where estimated in situ temperatures exceed 80°C ( 44 ), and transported with the ascending fluids and muds (see Supplementary Text). When the previously reported data in the Nankai Trough area (fig.…”

Mantle-derived fluids in the Nankai Trough Kumano forearc basin

“…Unusual light 7 Li/ 6 Li ratios in fluids recovered from a Kumano mud volcano (Nishio et al 2015) are in line with estimated temperatures of up to 310°C, indicating a deep lithium-bearing fluid source. Iodine ages of pore water samples from C0002 at depths as deep as 984 mbsf are significantly older than ages of the host sediments, Screaton et al 2009 andHammerschmidt et al, 2014). Subduction of the Philippine Sea plate occurs at a rate of 4.1 to 6.5 cm/yr.…”

Rheological behavior of a bentonite mud

“…A thumb rule in mud logging states that the drilling mud at the surface carries similar ratios of hydrocarbons in the drilled formations [2]. For in-stance, the ratios CH 4 /C 2 H 6 , CH 4 /C 3 H 8 , and CH 4 /C 4 H 10 imply rock formation variations and reservoirs productivities [3].…”