International Conference on Gas Hydrates (ICGH) (6th : 2008)


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PALEO HYDRATE AND ITS ROLE IN DEEP WATER PLIO- PLEISTOCENE GAS RESERVOIRS IN KRISHNA-GODAVARI BASIN, INDIA  Nishikanta Kundu, Nabarun Pal*, Neeraj Sinha, IL Budhiraja Reliance Industries ltd, Petroleum Business (E&P), RCP-5A, Ghansoli, Navi- Mumbai, India   ABSTRACT Discovery of natural methane hydrate in deepwater sediments in the east-coast of India have generated significant interest in recent times. This work puts forward a possible relationship of multi-TCF gas accumulation through destabilization of paleo-hydrate in Plio-Pleistocene deepwater channel sands of Krishna-Godavari basin, India. Analysis of gas in the study area establishes its biogenic nature, accumulation of which is difficult to explain using the elements of conventional petroleum system. Gas generated in sediments by methanogenesis is mostly lost to the environment, can however be retained as hydrate under suitable conditions. Longer the time a layer stayed within the gas hydrate stability zone (GHSZ) greater is the chance of retaining the gas which can be later released by change in P-T conditions due to sediment burial. P-T history for selected stratigraphic units from each well is extracted using 1-D burial history model and analyzed. Hydrate stability curves for individual units through time are generated and overlain in P-T space. It transpired that hydrate formation and destabilization in reservoir units of same stratigraphic level in different wells varies both in space and time. Presence of paleo hydrates is confirmed by the occurrence of authigenic carbonate cement and low-saline formation water. We demonstrate how gas released by hydrate destabilization in areas located at greater water depths migrates laterally and updip along the same stratigraphic level to be entrapped in reservoirs which is outside the GHSZ. In areas with isolated reservoirs with poor lateral connectivity, the released gas may remain trapped if impermeable shale is overlain before the destabilization of hydrate. The sequence of geological events which might have worked together to form this gas reservoir is: deposition of organic rich sediments → methanogenesis → gas hydrate formation → destabilization of hydrate and release of gas → migration and entrapment in reservoirs.  Keyword: Gas hydrates, bacterial gas, Krishna-Godavari basin   * Corresponding author: Phone: +91 022 447 70090 E-mail: NOMENCLATURE C1-n=Carbon number, M Cl-=Molar concentration of Chlorine= Pressure in Kg/cn2, GHSZ = gas hydrate stability zone, ρb= density g/cc, %Ro=Vitrinite reflectance, Ma= Million years, P-T=Pressure- Temperature, TH = Temperature in (ºC), TOC = Total organic carbon, o/oo = parts per thousand, φ=porosity. INTRODUCTION   Methane hydrate in deepwater sediments has generated interest in the recent years not only as a drilling hazard in deep water hydrocarbon exploration but also as a potential for huge energy resource. Hydrate has been reported from deep water sediments in Indian offshore in Bay of Bengal in Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  Figure 2: Genetic characterization of gas. Modified after Schoell (1984). Fig symbol: Star- interstitial gas, circle- reservoir gas. 1984 (Chopra, 1984) and since then several reports on occurrence of gas hydrate in the east coast basin of India have been published by various agencies (Rao et al., 1998, 2001). The most recent being the Indian National Gas Hydrate Program 2006-07 which established gas hydrates in several deep water locations in the eastern coast of India. The multi- trillion cubic feet super giant deep water Plio- Pleistocene gas discovery (Bastia, 2003, Bastia 2007) in Krishna-Godavari offshore the east coast of India by RIL generated huge interest within the geo- scientific community regarding its generation and entrapment. This work tries to explore a possible mechanism of migration and entrapment of the gas (bacterial) found in the Pliocene reservoirs through hydrate formation and its subsequent destabilization.  GEOLOGICAL SETUP  The Krishna-Godavari sedimentary basin is a major intracratonic rift within Gondwanaland until Early Jurassic overlain by the pericratonic basin of Late Jurassic and Early Cretaceous (Biswas, 1999; Rao, 2001; RIL unpublished report). This basin covers the shelf and slope in the eastern passive continental margin of India. The basin evolved through rifting and subsequent drifting during Mesozoic. The Mesozoic and subsequent Early Tertiary deep water sequence provide the base for the Late Tertiary marine sediments brought in by either large canyon systems or feeder channels in the upper slope. Sediment input in this region has been dominated by the Krishna and Godavari river systems. The basin has several gas discoveries in the deep water Plio-Pleistocene multi-stacked, sinuous channel-levee complexes.  The present study is focused mainly in the upper-slope region of the Godavari river mouth. Two drilled wells A, B and a pseudo well C, along the cross section A-A’ is taken for detailed analysis (Fig.1).   GAS COMPOSITION  Gas composition in the study area shows dominance of methane (>99%) with minor amounts of higher hydrocarbons. The C1/(C1-C5) ratio in all the analyzed gas samples collected during MDT and DST varies from 0.9923-0.9995 which indicates a predominantly bacterial origin. Results of interstitial gas analysis from shallow cores in the sea-bed near the study area also indicate biogenic nature of seeped gas. Samples were also analyzed for their δ13C methane values. Methane δ13C varies from -69.9 to - 71.6 o/oo in the reservoir gases and 61.2 to -105.4 o/oo 0 5 10 15 20 25 30 -20 -30 -40 -50 -60 -70 -80 Post mature  dry gas Post mature  wet gas Matured Gas formed with oil Mixed Gas Bacterial Gas  δ13 C m et ha ne (0 / 00 ) Gas Wetness (%) Figure 1: Location map. A-A’: X-section along well A, B and C A     A’  30 25 20 15 10 5 0 -5 0 5 10 15 20 25 30 95.9 % C1 90.4% C1 90.4 % C1 Base of GHSZ100% C1  Temperature (oC) P re ss ur e (M P a) Seafloor 100% C1 Base of GHSZ Geothermal gradient Figure 3: Plot of methane hydrate stability relationship using different gas composition. Red Line: stability curve derived using gas composition and salinity of the study area (Kamath and Holder, 1987); Blue line: gas stability relationship following Handa (1990); Green curve: Curve calculated using CSMHYD hydrate program (Solan 1998) for 95.9 % methane; Pink Line: Curve calculated using CSMHYD hydrate program (Solan 1998) for 90.4 % methane; Sea floor depth for well B at study area with measured Geothermal gradient and position of GHSZ) for different gas compositions. in the interstitial gas. Genetic gas characterization diagram (after Scholl, 1983) suggests primarily bacterial origin for reservoir and interstitial gas from the cores (Fig 2).  It is established that bacterial gas is generated at low temperature through decomposition of organic matter by anaerobic microorganisms from thermally immature source rocks (Dudley and Claypool, 1981). The two most important mechanisms of methane generation are CO2 reduction and acetate fermentation. In marine sediments methane is produced mainly during CO2 reduction by hydrogen while the acetate fermentation is dominant in fresh water deposits. There are over 60 different species of methanogens reported to be responsible for the generation of bacterial gas. Though these species can survive in a wide rage of temperatures (2-100oC) however the majority thrive within a narrow range of 35-45oC. The factors that control the level of methane production after sediment burial are anoxic environment, sulfate deficient environment, suitable temperature, salinity (< 4M Cl-) and availability of organic matter. Thus the depth of significant production of bacterial gas depends on the local geothermal gradient and rate of sedimentation which may vary from basin to basin and over time within a single basin (Shurr and Ridgley, 2002).  Biogenic gas in deep waters  Source of this huge bacterial gas, hosted in Plio-Pleistocene submarine channel-levee complexes in Krishna-Godavari deep waters is difficult to explain using the elements of conventional petroleum system. Although considerable work is published on biogenic gas that has been generated and trapped in shelf and deltaic depositional systems, little is understood about this type of deposits in distal deepwater settings. One possible explanation of such an accumulation can be generation of gas by bacterial activity in deep water sediments and their concentration by formation of gas hydrate under suitable conditions. On subsequent sedimentation these gas hydrate may become unstable on reaching higher temperatures and thereby releasing huge amount of gas. In this paper possibility of the formation of hydrate at various stratigraphic layers and their subsequent destabilization to form large gas reservoirs are discussed in the following sections.  HYDRATE FORMATION  Formation and occurrence of gas hydrate are controlled by temperature, pressure, gas chemistry, pore-water salinity, availability of gas and water, migration pathways, and presence of reservoir rocks (Collett, 1995, 2001). Gas hydrate stability curve is generated using equation 1 (Kamath and Holder, 1987), considering  Figure 4: P-T history of 3 representative layers from well B. The Pleistocene layer is within the GHSZ all through out its history; the reservoir sand (Broweri) was within the GHSZ from 2.05 to 1.3 Ma and was free of hydrates from 1.3 Ma to present day; Abies layer started depositing from 3.58 Ma and have never entered within the GHSZ  gas chemistry and pore water salinity in the drilled wells of the field (figure 3): 885.27ln8186.8 −= PTH    (1) In addition hydrate stability diagram by Handa (1990) and CSMHYD hydrate program by Solan (1998) is overlain in Figure 3. In the discussed diagram the base of GHSZ for different gas compositions, at a representative location (well B) of water depth 1277m with measured geothermal gradient of 4.5oC/100m is shown. It is observed that the GHSZ is depressed with increase in higher hydrocarbon content. During drilling of the well A and B, presence of hydrate was confirmed between depths 1400 to 1425m based on the distinctive signatures in resistivity, and sonic logs. The observed result is in conjunction with the theoretical predicted GHSZ taking 100% methane in the gas.  BURIAL HISTORY AND ESTIMATION OF PALEO P-T CONDITIONS  One dimensional burial history modeling is carried out in 3 locations using PETROMOD 1DTM (v 9.0) software of IES, Germany. For calibration of the models, EASY %Ro of Sweeney and Burnham (1990) was used and compared with the measured data. The measured formation temperatures (from drill stem tests (DSTs), the extrapolated bottom hole temperatures (BHTs) using the Horner plot correction method were used to constrain the model. Detailed age subdivisions of different units were made by integrating the biostratigraphic, seismic, well logs and sedimentological data (Mandal et al., 2005). Several condensed sections were identified in the Plio-Pleistocene section which is recognized by the abundance and diversity of microfossils (nannofossils and foraminifera), presence of typical stratigraphic marker microfossils. The identified condensed sections were precisely dated based on global nanno fossil datum such as D. berggrenii (6.0Ma), S. abies (3.58 Ma), D. brouweri (2.25 Ma) and H. Sellii (1.47Ma). Further each individual unit is subdivided on the basis of major lithotypes and corresponding ages were assigned. Shale compaction curve is calculated for each individual wells from the bulk density using an appropriate grain density for mud rock, namely 2.7g/cc. The general equation for fractional porosity used: 7.1 7.2 bρ−=Φ      (2) Depth-porosity curves were obtained by fitting a polynomial curve to the porosity data which was calculated from smoothed density log (ρb) using equation 2.  The MFS age are compared with the eustatic sea level curve of Haq et al., (1987) and was adjusted to the time scale of Berggren et al (1995). Water depth at various age intervals was calculated using biostratigarphy from the wells in conjunction with the standard sea level curves. Sediment water interface temperature was calculated for different age intervals using the model given by Wygrala et al (1989). For the reconstruction of the thermal histories a heat flow of 75mW/m2 was used for the rifting phase. After that a gradual cooling is assumed as proposed by theoretical models (e.g. McKenzie, 1978) with present day surface heat flow around 55mW/m2. Burial history curves for the discussed wells are computed and calibrated with measured BHT and DST temperatures.  0 10 20 30 40 50 60 70 28 24 20 16 12 8 4 0 0.0 Ma 0.0 Ma  Pr es su re  (M Pa ) Temperature (oC) 0.8 Ma Pleistocene Broweri sand Abies 3.58 Ma2.05 Ma 1.3 Ma 0.0 Ma Hydrate stability curve  RESULTS AND DISCUSSION  Pressure and temperatures variations with time of each individual unit are extracted from the calibrated 1D burial history model for different wells. In well B the condensed section abies was deposited between 5.3 to 3.58 Ma. The section between abies- pentaradiatus, pentaradiatus-brouweri, brouweri-sellii and sellii-recent are deposited between 3.58-2.46, 2.46-1.95, 1.95-1.47 and 1.47-0 Ma respectively. Pressure-Temperature data for three representative stratigraphic layers at this well are plotted together with the gas hydrate stability diagram (Fig 4). From the figure it is observed that the abies section has never been within the hydrate stability field, while the brouweri reservoir sand was within the GHSZ from 2.05 to 1.3 Ma and free of hydrate after that till present day. However, the Pleistocene layer is within the GHSZ all through out its history. This model of the stability relations matches with the present day hydrate occurrence in the Pleistocene section evidenced from well logs. Though P-T conditions as described above support formation of gas hydrate and their destabilization at different times, the actual hydrate formation will depend on the availability of enough methane and water within the pore spaces. Availability of methane depends on the degree of methanogeneis of the recent sediments and/or supply from matured source rocks from deeper layers. In this field the measured TOC content of the various litho units varies from 1.5-2 % which is conducive for bacterial activity to produce biogenic methane. When the P-T condition is favorable, this gas together with the available water will produce gas hydrate, there by retaining a part of the gas into the sediments. The thickness of the hydrate layer depends on the amount of time it had remained within the stability field which is mainly controlled by the sedimentation rate of the overlying units.  In order to understand the spatio-temporal variation of hydrate formation and its destabilization, a representative 2D seismo-geological section along AA’(Fig 5) has been considered.  The water depth varies from 400m in the NW part to more than Figure 5. Seismogeological section through wells A, B and C. BSR (Bottom simulation reflector) is shown.  Figure 6: P-T history of reservoir sand (Broweri) encountered in 3 wells along the given section AA’. The reservoir sand was within the Hydrate stability field (GHSF) till 1.85 Ma, 1.3 Ma and 1.18 Ma in well A, B and in C respectively. Hydrate destabilization in deeper wells will charge hydrate free sand reservoirs in the upslope locations under suitable entrapment conditions. 1800m in the SE part. The drilled A & B wells are situated at water depth of 1044 and 1277m respectively. The gas bearing reservoir sand of D. brouweri age encountered at A & B locations at a depth 2011 and 2190m respectively. A pseudo 1D burial model at location C is considered at a water depth of 1786m in the SE part. This helps in visualizing the migration and entrapment of gas in the reservoir sands via hydrate destabilization.  P-T history of reservoir sand (D brouweri) at locations A, B and C are plotted in gas hydrate stability field (Fig. 6). It is observed that the reservoir sands have stayed more than 10, 36 and 42 % of their existence within the hydrate stability field respectively. The sediments within and below these units also have adequate TOC (>2%), therefore the chances of forming adequate thickness of hydrate layers in these units was very high. Due to continuous sedimentation over the hydrate layer the P-T conditions changed and ultimately the hydrate destabilized to release the entrapped gas. The reservoir sands up-dip of well-A are free of hydrate when stratigrafically equivalent layer in down-dip locations are still within the GHSZ. When these hydrate destabilize gas is released part of which is lost and the remaining may migrate up dip laterally depending on the shale-sand ratio of the carrier beds. It is observed that for a mean sand percentage of 20- 50% a large lateral migration of fluid flow develops as sand bodies attract flow from a region 2 to 7 times the radius of the sand body itself (Glenzen and Lerche, 1985). In the discussed gas-field the presence of aerially extensive thin beds in the levee/ interchannel areas is established through conventional cores and high resolution seismic attribute analysis. These thin beds favor lateral migration of released gas from hydrate destabilization. However, the amount of methane available for entrapment depends on the thickness of destabilizing hydrate layer, timing of gas release and timing of trap development. The chance of charging channel-levee sands from destabilized hydrate in the study area is thus quite high.  Hydrate melting is generally associated with release of low saline water and precipitation of carbonates (Matsumoto, 1989; Zhu et al 2003). Presence of authigenic carbonate cement (mostly siderite) in reservoir sands of wells A and B provides further evidence for the destabilization of gas hydrate. The above results thus confirm the presence of paleo gas hydrate and their destabilization at different geological time. Migration (both lateral and vertical) and entrapment of the released gas at favorable conditions has resulted in the huge gas reservoir in the deepwater Godavari basin.   CONCLUSION  The gas in the deep water Plio-Pleistocene channel-levee complex in the Godavari basin in the east coast of India is predominantly bacterial in nature. Our study furnishes a possible explanation of this large gas accumulation by the destabilization of the paleo hydrate. The following sequence of geological events might have worked together to form this gas reservoir in the following sequential order: deposition of organic rich sediments in deep water → bacterial activity at reducing environment (methanogenesis) → formation of gas hydrate → -5 0 5 10 15 20 25 30 35 40 45 50 30 25 20 15 10 5 0 0.0Ma 0.0Ma 0.0Ma 2.05Ma 1.15Ma Well C 2.05Ma Well B  Pr es su re  (M Pa ) Temperature (oC) 2.05Ma 1.47Ma 1.15Ma Well A 1.85Ma 1.3Ma 1.18Ma Hydrate stability curve  increase of temperature leading to melting of hydrate and release of gas →  migration and subsequent entrapment in porous and permeable sand bodies.  ACKNOWLEDGEMENTS  The authors would like to thank Reliance Industries Limited for granting permission to publish and present this work. We would also like to put our sincere thanks to all the members of the interpretation group of the KG-D6 block for their valuable support at various stages of this work.   REFERENCES   Bastia R, Singh RJ, Sinha N. 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