Subsurface Occurrence of Natural Gas Hydrate in the Nankai Trough Area: Implication for Gas Hydrate Concentration

Abstract. The Nankai Trough runs along the Japanese Islands, where extensive BSRs have been recognized in its forearc basins. High resolution seismic surveys and site-survey wells undertaken by the MITI have revealed the gas hydrate distribution at a depth of about 290 mbsf. The MITI Nankai Trough wells were drilled in late 1999 and early 2000. The highlights were successful retrievals of abundant gas hydrate-bearing cores in a variety of sediments from the main hole and the post survey well-2, keeping the cored gas hydrate stable, and the obtaining of continuous well log data in the gas hydrate-dominant intervals from the main hole, the post survey well-1 and the post survey well-3. Gas-hydrate dominant layers were identified at the depth interval from 205 to 268 mbsf. Pore-space hydrate, very small in size, was recognized mostly filling intergranular pores of sandy sediments. Anomalous chloride contents in extracted pore water, core temperature depression, core observations as well as visible gas hydrates confirmed the presence of pore-space hydrates within moderate to thick sand layers. Gas hydrate-bearing sandy strata typically were 10 cm to a meter thick with porosities of about 40 %. Gas hydrate saturations in most hydrate-dominant layers were quite high, up to 90 % pore saturation.
All the gas hydrate-bearing cores were subjected to X-ray CT imagery measurements for observation of undisturbed sedimentary textures and gas-hydrate occurrences before being subjected to other analyses, such as (1) petrophysical properties, (2) biostratigraphy, (3) geochemistry, (4) microbiology and (5) gas hydrate characteristics.     

Overview of the MITI Nankai Trough Wells: A Milestone in the Evaluation of Methane Hydrate Resources

Abstract. Bottom-simulating reflectors suggestive of the presence of methane hydrates are widely distributed below the ocean floor around Japan. In late 1999, drilling of the MITI Nankai Trough wells was conducted to explore this potential methane hydrate resource and a Tertiary conventional structure. The wells are located in the Northwest Pacific Ocean off Central Japan at a water depth of 945 m. A total of six wells were drilled, including the main well, two pilot wells, and three post survey wells at intervals of 10–100 m. All wells except the first confirmed the occurrence of hydrates based on logging-while-drilling, wire-line logging and/or coring using a pressure and temperature coring system in addition to conventional methods. Based on the various well profiles, four methane hydrate-bearing sand-rich intervals in turbidite fan deposits were recognized. Methane hydrates fill the pore spaces in these deposits, reaching saturation of up to 80 % in some layers. The methane hydrate-bearing turbiditic sand layers are less than 1 m thick, with a total thickness of 12–14 m. The bottom depth of high hydrate concentration correlates well with the depth of the bottom-simulating reflector. Based on these exploration results, the Japanese government inaugurated a 16-year methane hydrate exploitation program in 2001.     

Petrophysical Properties of Natural Gas Hydrates-Bearing Sands and Their Sedimentology in the Nankai Trough

Abstract. The Nankai Trough parallels the Japanese Island, where extensive BSRs have been interpreted from seismic reflection records. High resolution seismic surveys and drilling site-survey wells conducted by the MTI in 1997, 2001 and 2002 have revealed subsurface gas hydrate at a depth of about 290 mbsf (1235 mbsl) in the easternmost part of Nankai Trough. The MITI Nankai Trough wells were drilled in late 1999 and early 2000 to provide physical evidence for the existence of gas hydrate. During field operations, continuous LWD and wire-line well log data were obtained and numerous gas hydrate-bearing cores were recovered. Subsequence sedimentologic and geochemical analyses performed on the cores revealed important geologic controls on the formation and preservation of natural gas hydrate. This knowledge is crucial to predicting the location of other hydrate deposits and their eventual energy resource. Pore-space gas hydrates reside in sandy sediments from 205 to 268 mbsf mostly filling intergranular porosity. Pore waters chloride anomalies, core temperature depression and core observations on visible gas hydrates confirm the presence of pore-space hydrates within moderate to thick sand layers. Gas hydrate-bearing sandy strata typically were 10 cm to a meter thick. Gas hydrate saturations are typically between 60 and 90 % throughout most of the hydrate-dominant sand layers, which are estimated by well log analyses as well as pore water chloride anomalies.
It is necessary for evaluating subfurface fluid dlow behavious to know both porosity and permeability of gas hydrate-bearing sand to evaluate subsurface fluid flow behaviors. Sediment porosities and pore-size distributions were obtained by mercury porosimetry, which indicate that porosities of gas hydrate-bearing sandy strata are approximately 40 %. According to grain size distribution curves, gas hydrate is dominant in fine- to very fine-grained sandy strata.     

The Geochemical Context of Gas Hydrate in the Eastern Nankai Trough

Abstract. Geochemical studies for gas hydrate, gas and organic matter collected from gas hydrate research wells drilled at the landward side of the eastern Nankai Trough, offshore Tokai, Japan, are reported. Organic matter in the 2355 m marine sediments drilled to Eocene is mainly composed of Type III kerogen with both marine and terrigenous organic input. The gas hydrate-bearing shallow sediments are immature for hydrocarbon generation, whereas the sediments below 2100 mbsf are thermally mature. The origins of gases change from microbial to thermogenic at around 1500 mbsf.
Carbon isotope compositions of CH₄ and CO₂, and hydrocarbon compositions consistently suggest that the CH₄ in the gas hydrate-bearing sediments is generated by microbial reduction of CO₂. The δ¹³C depth-profiles of CH₄ and CO₂ suggest that the microbial methanogenesis is less active in the Nankai Trough sediments compared with other gas hydrate-bearing sediments where solid gas hydrate samples of microbial origin were recovered. Since in situ generative-potential of microbial methane in the Nankai Trough sediments is interpreted to be low due to the low total organic carbon content (0.5 % on the average) in the gas hydrate-bearing shallow sediments, upward migration of microbial methane and selective accumulation into permeable sands should be necessary for the high concentration of gas hydrate in discrete sand layers.     

BSRs and Associated Reflections as an Indicator of Gas Hydrate and Free Gas Accumulation: An Example of Accretionary Prism and Forearc Basin System along the Nankai Trough, off Central Japan

Abstract. Multi-channel seismic data obtained from the Nankai accretionary prism and forearc basin system has been studied to elucidate the migration and accumulation process of gas to the BGHS and examine the distribution pattern of BSRs and characteristic reflections associated with them.
BSRs are distributed widely in the Nankai accretionary prism and associated forearc basins (33,000 km²) and 90 % of them have migration and recycling origins. The widest distribution of the BSRs can be seen at the prism. A correlation between the BSR distributions and prism size shows that the BSRs tend to be more well-developed in a prism of large size. This suggests that a large prism may produce much amount of gas-bearing fluids that migrate to the BGHS and form the BSRs (tectonic control), hi the forearc basins, the BSRs are identified at topographic highs, anticlines and basin margins (structural control).
The upward migration of gas-bearing fluids is carried out through permeable sand layers and as a result, the distribution of BSRs is confined to alternating beds of sand and mud facies (sedimentary control). However, if there is enough time for upward migration and accumulation of gas to the BGHS, the BSRs can be generated widely in low-permeable mud facies (time control).
Those results imply that structural, tectonic, sedimentary and time controls are primary factors to decide the distribution of BSRs in the Nankai Trough area.     

Application of AVO Analysis to Seismic Data for Detection of Gas below Methane Hydrate Stability Zone in Nankai Trough Area

Abstract. For the purpose of development of methane hydrate, occurring in the deep marine subsurface, as a resource, the most important issue is to understand the methane hydrate system (generation, migration and accumulation) as well as to delineate the methane hydrate reservoir properties. We have applied the Amplitude Versus Offset (AVO) analysis to the seismic data acquired in the Nankai Trough, offshore Japan, in order to confirm the occurrence of gas just below the methane hydrate-bearing zone, assuming that gas will show a so-called Class-3 AVO response. Knowledge of the amount and occurrence of gas in the sediment below methane hydrate-bearing zone is one of the keys to understand the methane hydrate system.
We have utilized the qualitative analysis of AVO methodology to delineate how gas is located below the BSR, which is thought to be the reflection event from the interface between the methane hydrate-bearing zone and the underlying gas-bearing zone. In the region of MITI Nankai Trough Well PSW-3, we observe two BSRs separated by 25 ms. After AVO modeling using well data, we applied AVO attribute analysis and attribute crossplot analysis to the seismic data. Finally we applied an offset-amplitude analysis to CMP gather data at specific locations to confirm the results of AVO attribute analysis. The AVO analysis shows that there is very little gas located in the underlying sediment below methane hydrate-bearing zone. This result supports the fact that we could not obtain any clear evidence of gas occurrence just below the methane hydrate-bearing zone in the Nankai Trough well drilling.     

Geochemical Process of Gas Hydrate Formation in the Nankai Trough Based on Chloride and Isotopic Anomalies in Interstitial Water

Abstract: Interstitial water expelled from gas hydrate-bearing and -free sediments in the Nankai Trough are analyzed in terms of Cl-, SO₄²-, δ¹⁸O and δD. The baselines for the Cl- concentration and δ¹⁸O value are close to seawater values (530 mM and 0%), indicating that the interstitial water is of seawater origin. The δD values decrease with depth, implying isotopic exchange of hydrogen between upwelling biogenic methane depleted in D and interstitial water. The Cl- concentrations in gas hydrate-bearing sediments are anomalously low, while the δ¹⁸O and δD values are both high, suggesting that the water forming these gas hydrates was poor in Cl- and enriched in ¹⁸O and D during gas hydrate formation. Calculation of the gas hydrate saturations using Cl "and δ¹⁸O anomalies gives results of up to 80 % in sand, and shows that the δ¹⁸O baseline is not consistent with the Cl" baseline. The δ¹⁸O baseline increases by +1% in gas hydrate-free clay and silt. This is considered to be caused by clustering of water molecules after gas hydrate dissociation in response to the upward migration of the base of gas hydrate stability, as indicated by the presence of a double bottom-simulating reflector at this site. The water clusters enriched in ¹⁸O are responsible for the increase in the δ¹⁸O baseline with normal Cl". The abrupt shallowing of the base of gas hydrate stability may induce the dissociation of gas hydrates and the accumulation of gases in the new stability zone, representing a geological process that increases gas hydrate saturation.     

注蒸汽开采冻土区天然气水合物数值计算与野外应用

试开采研究是天然气水合物从理论研究走向商业化开采的必经之路。提出的注蒸汽开采是一种综合开采方法,是在降压开采的基础上,往孔内注入热蒸汽对水合物目标开采层进行激振往复式热激发。注蒸汽开采能够避免“自保护效应”,促进水合物进一步分解,理论上能够扩大开采范围、提高产气量。利用FLUENT数值模拟软件对蒸汽加热水合物层动态过程进行数值计算,通过模拟计算结果对比分析,在满足开采要求的前提下确定最佳的注蒸汽功率为20 kW,注热时长为38 h。在青海木里盆地天然气水合物试采项目中,注蒸汽开采进行了5.2 h开采试验,产气量为3.25 m3。     

TOPKAPI模型的改进及其在淮河流域洪水模拟中的应用研究

TOPKAPI模型是一个以物理概念为基础的分布式流域水模型。对TOPKAPI模型进行了改进，增加了植物截留、降水下渗、土壤水深层渗漏、地下径流、河川径流等计算模块，并对壤中流计算模块作了相应的调整，将TOPKAPI模型改进成为一个较完整的分布式流域水模型。采用因特网(Internet)上免费提供的数字高程、土壤和植被类型等网格资料，选用流域面积约为10000km^2的中国淮河息县以上流域，应用改进后的TOPKAPI模型进行了洪水模拟，模型成果令人满意。     

地下水水平井流的模型及数值模拟方法—考虑井管内不同流态

评述用线汇刻画水平井管可能存在的问题, 并用理想模型证明.提出考虑井管不同流态(线性流与非线性流) 水流阻力的水平井流的模型, 并将内边界取在水平井的出水口处, 避免了采用线汇刻画水平井管时假定其流量分布和水头分布的困难.运用作者提出的等效渗透系数的概念与确定方法, 将水平井-含水层系统视为含有圆柱形透镜体的非均质含水层, 给出新的水平井流的数学模型.求解了一个理想模型, 得出河下水平井开采地下水的动态.      

岩浆侵入体热传导模型优选及应用

在对比分析不同侵入体热传导模型的基础上,进一步结合Easy Ro%模型对前人报道的受岩浆侵入体影响的实测Ro数据进行了数值模拟,结果表明改进的Fjeldskaar模型具有较好的应用前景。同时侵入体不同初始温度及不同厚度对围岩热演化程度影响的数值模拟结果表明,岩浆侵入体的热作用范围有限,不同地质条件下,影响的范围不同。其中:侵入体初始温度越高,影响范围越大;侵入体厚度越大,影响范围(X/D值)也越大,但是一般X/D<2(X/D代表侵入体与接触面的距离和侵入体厚度的比值)。侵入体热传导模型的初步应用表明,岩浆作用带来的热源对烃源岩生烃进程产生了重要影响,可以加速烃源岩的成熟,使得生烃期提前。     

天然气甲烷碳同位素动力学研究及其应用:以塔里木盆地库车坳陷克拉2气田为例

通过有压力的黄金管封闭体系生烃模拟实验和GC-IRMS测定,结合GOR-Isotope Kinetics专用软件,求取了塔里木盆地库车坳陷三叠系-侏罗系烃源岩生成甲烷的碳同位素动力学参数。结合地质背景,探讨了克拉2气田天然气的成因。克拉2气田天然气主要来源于早中侏罗世煤系烃源岩,属阶段捕获气,为-5Ma以来的天然气聚集,对应成熟度范围Ro为1．3％-2．5％。在此基础上,建立了克拉2气田天然气运聚成藏动力学模式,从而为天然气定量评价和动态研究提供了新思路。