Seismic Reservoir Characterization Using Model Based Post-Stack Seismic Inversion: in Case of Fenchuganj Gas Field, Bangladesh
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Journal of the Japan Petroleum Institute, 59, (6), 283-292 (2016) 283 [Regular Paper] Seismic Reservoir Characterization Using Model Based Post-stack Seismic Inversion: In Case of Fenchuganj Gas Field, Bangladesh Shefa Ul KARIM†1), Md Shofiqul ISLAM†1)＊, Mohammad Moinul HOSSAIN†2), and Md Aminul ISLAM†3) †1) Dept. of Petroleum and Mining Engineering, Shahjalal University of Science and Technology, Sylhet 3114, BANGLADESH †2) Geophysical Div., Bangladesh Petroleum Exploration and Production Co., Ltd., Dhaka, BANGLADESH †3) Dept. of Petroleum Geoscience, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, NEGARA BRUNEI DARUSSALAM (Received August 31, 2015) In the study, model based post-stack inversion technique was used to create pseudo logs at each seismic trace at the well location to constitute high resolution acoustic inverted impedance models. Interpretation of GR log, SP log, Caliper log, and seven sand bodies were marked as reservoir zones in well FG #X, which were identified as a hydrocarbon bearing reservoir. All of predicted gases bearing zones in the well FG #X show the low acoustic impedance (AI) values in inverted section analogous with the calculated AI value of logging data. The imped- ance value in an inverted section during 1660-1980 ms represents an image of the alteration of thin sand and a thick shale layer of Upper Bhubon formation. By observing the relatively lower AI values in the inverted section three locations have marked as additional well locations (PW 1, PW 2 and PW 3), which are more prospective for optimizing the gas recovery from this field. Keywords Surma Basin, Wavelet, Reservoir, Seismic trace, Post-stack inversion, Acoustic impedance 1. Introduction let. Also, due to the band limited nature of the seismic data, lack of low frequencies prevent the transformed Reservoir characterization plays an essential role in impedance trace from gaining the basic impedances or the prediction of the reservoir properties as well into the velocity structure, which is crucial to making a geologi- economic potential of the field. A detailed study of cal interpretation5). An attempt to recover this resolu- the static behavior of a producing hydrocarbon reser- tion is usually made by obtaining the reflectivity voir is essential for the development planning of the through a deconvolution operation, which is an inverse reservoir and to reduce risk and uncertainty in choosing problem6). new drilling locations1),2). Perception of reservoir Seismic inversion is the process of extracting infor- characterization requires integrated analysis and under- mation about elastic rock properties from seismic data standing of the available data, such as seismic data and based on the travel-time, amplitude, and phase informa- well log data. tion contained within a seismogram7). It is an optimal In any seismic reservoir characterization studies, the way to get a better subsurface image1),5),8)～11). first step towards a successful hydrocarbon discovery is Commercially, different seismic inversion methods are the mastering of a good subsurface image of seismic used to map the detailed reservoir properties such as data. The reflected seismic wave amplitudes are func- lithology and fluid properties5),12). In this research tions of acoustic/elastic impedances, which project a work, model based post-stack seismic inversion analy- contrast (reflection coefficient) between the lithology sis is used to identify the gas bearing potential zones above and below a reflecting boundary1)～4). Seismic and possible well locations for further development of data is the output of a convolution operation between the Fenchuganj gas field. Only a small part of full res- the earth reflectivity and a source wavelet. The convo- ervoir and modified coordinates were studied due to lution operation produces a band-limited trace, the data confidentiality. The Fenchuganj field has several bandwidth of which is determined by the seismic wave- pay zones and stratigraphy of the structure is consists alternate shale and sandstone in varying proportion13). DOI: dx.doi.org/10.1627/jpi.59.283 A conventional seismic interpretation technique is very ＊ To whom correspondence should be addressed. uncertain in predicting rock physics. For this reason, ＊ E-mail: sho_fi[email protected] this study intends to reduce the uncertainty to a delinea- J. Jpn. Petrol. Inst., Vol. 59, No. 6, 2016 284 arine, shallow-marine sandstones, siltstones, and shales that contain abundant plant-derived organic materials. The variety of sedimentary facies of the Surma Basin indicates a range of depositional environment during Neogene time16). These strata generally contain about 0.5 to 3 % total organic carbon (TOC) although in places the content of organic matter may range up to 10.5 %. Thermal maturation is sufficient to generate natural gas and liquids throughout much of the area15). The Fenchuganj gas field’s geology is similar to that of other fields situated in Surma Basin. The stratigra- phy sediments of Fenchuganj structure consists alter- nate shale and sandstone in varying proportion of Oligocene to Recent age17). The stratigraphic succes- sion of the Fenchuganj gas field is based on geological data, seismic data, and well data with brief lithological description are given in Table 1. The reservoirs have been founded in the Miocene sediments, which are mainly composed of alternating gray to dark gray clay, Surma Basin is same as Sylhet Trough (after)15). Sediment thickness very fine to medium grained sandstones and potential in the map is shown in meter. source rocks include shales and carbonaceous shales of Eocene, Oligocene, and Miocene age in the basin cen- Fig. 1● Generalized Geological Map of Bangladesh and Adjoining ter and in the synclinal troughs between the fold Area trends13),18). tion of hydrocarbon bearing distribution prediction. 3. Methodology 2. Geology and Stratigraphy The seismic inversion method is basically a process of transforming seismic amplitude value to impedance The study area is under the Fenchuganj structure value. This is done by deconvolution process which which is situated in the transition zone between the transforms seismic trace to earth reflectivity. Inversion central Surma Basin and the folded belt in the east and is the subsurface modeling technique to produce a geo- is closest to the eastern margin of the central Surma logic structure using seismic data as input and well data Basin, which is separated in the north from the as control19). The post-stack seismic inversion methods Kailashtila Anticline, in the east from the Harargaj use stacked (zero-offset) seismic data to produce images Anticline, and in the south from Batchia Anticline of the AI in depth or time. AI is one of rock-physics (Fig. 1)13). In the Surma Basin (Sylhet Trough), it parameters, which is influenced by the type of lithology, appears that the Fenchuganj structure is the third elevated porosity, fluid content, depth, pressure and tempera- structure, followed by the Chattak and the Atgram ture20). Anticline14). This structure appears as a reversibly The fundamental concept of seismic exploration is to faulted asymmetrical anticline with a NNE-SSW trend- send a short time signal into the earth, which is then re- ing axis. This fault is wider in the southern region, flected back from a boundary between two units called which becomes narrower towards north. It observed reflector. The signals are transmitted through the earth that the eastern flank of the anticline is steeply dipping as an elastic wave and brought back to the receiver sub- than the western flank due to reverse fault13),14). surface information such as geological structure, lithol- Structural and combination traps of Miocene age ogy and fluid through travel time, reflection amplitude occur along stratigraphic boundaries, in sandstone-filled and phase variation20). If we assume that the angle of channel deposits, and in sandstone beds sealed laterally incidence is zero and that the layers are flat, the by shale-filled channels; these comprise major traps in Zoeppritz equations will simplify to the more manage- the eastern part of the basin. In general, these sedi- able equation given by Eq. (1). mentary strata have been folded into several large-scale ZZii+1 − anticlines that are unfaulted or slight to moderate fault ri = （1） ZZii+1 + in the western and central parts of the basin15). The Surma Basin contains a great thickness of Where ri is the zero-offset P-wave reflection coefficient Tertiary sedimentary strata. This basin contains as at the i-th interface of a stack of n-th layer, and Zi＝ρi× much as 20 km of sediments consisting of deltaic, estu- Vi is the AI impedance of the i-th layer. J. Jpn. Petrol. Inst., Vol. 59, No. 6, 2016 285 Table 1 Lithostratigraphic Succession of the Fenchuganj Gas Field Age Formation Depth [m] Thickness [m] Lithology Recent Alluvium 0-30 30 Unconsolidated sand, silt and clay. Late Pliocene DupiTila 30-298 268 Mostly sandstone and minor clay. sandstone: brown to light brown and coarse. clay: dark gray, very soft and sticky. Middle Pliocene Tipam 298-1150 852 Mostly sandstone and minor clay. sandstone: light to off white, medium grained. clay: gray to dark gray, soft to moderately hard. Miocene Upper Bokabil 1150-1466 316 Shale: gray to bluish gray, soft to moderately hard and compact, and also laminated. Middle Bokabil 1466-1766 300 Sandstone and shale alteration. sandstone: light to clear and medium to fine. shale: laminated, silty, moderately hard. Lower Bokabil 1766-2236 470 Mostly shale with minor sandstone. shale: gray to dark gray, hard and compact. sandstone: light colored and fine to very fine. Early Miocene Upper Bhuban 2236-4977 914-2741 Alternation of sandstone and shale, with minor calcareous (vary) siltstone. sandstone: light to clear and fine grained. shale: gray to dark gray, hard and compact. Data source: Geology Div., Bangladesh Petroleum Exploration and Production Co., Ltd., Dhaka. Equation (1) can be used as a simplified model for the seismic is added the logging data with stacked seis- the reflections found on a stacked seismic section.