| The current research aims to present the sand-shale facies distribution and their paleoenvironments in the C-sand interval of Lower Goru Formation of Sawan gas field,Middle Indus Basin,Pakistan.The data used for the classification of facies analysis incorporates the integration of well-log,3D post stacked seismic data,information of core data from the published material,and recent production data.In the study area,fifteen wells are drilled since2003,and the C-sand(marked as X1 horizon within the study)interval of the Lower Goru formation is proven reservoir and has vast potential of the gas reservoir.The Sawan Gas Field is one of the most promising gas fields of Middle Indus Basin in Pakistan with a cumulative production of 850 BCF.In the reservoir interval(C-sand),the thin shale sequence is interbedded and dispersed resulting in extreme heterogeneity.Consequently,conventional seismic amplitude interpretation fails to delineate the sand-shale facies distribution.In this case study,the depositional facies of the reservoir C-sand interval with the integration of seismic facies analysis,facies modeling using stochastic technique,electrofacies analyses,recent production data analysis with 3D post stacked seismic attribute analysis,sweet spot evaluation,automatic fault extraction using artificial ant-tracking,and constrained sparse spike inversion for reservoir characterization were analyzed for prospect evaluation and field development plan to achieving maximized production.The thorough seismic interpretation of the seismic section and marked horizons shows that the targeted area of C-sand interval has fewer discontinuities and stratigraphic traps are present in the study area which is the main reason for the accumulation of the reservoir sandstone.Integrated seismic analysis revealed that the C-sand interval has parallel to sub-parallel reflections,strong reflection amplitude,medium refection continuity,and medium reflection frequency and has an external wedge-shaped geometry.The basin slope and fill geometry are defined as onlap.Numerous stratigraphic up-dip pinch-out were recognized on the seismic section.An NNW-SSE oriented fault(named as Sawan fault)passing near the right side of Sawan-01 well divides the study area into east and west reservoir compartments.The study area lies on the right side of the fault.The throw of this fault is minor.The strong amplitude reflection patterns around 2.175 second suggest the sandstone reservoir facies.Sedimentary and log facies analysis revealed that the reflection pattern associated with medium to high continuity,medium to low frequency and strong amplitude represents the interbeds of sandstone with mudstone and siltstone sediments which were deposited under high energy conditions with trough crossbedding,and indicates the fluvial-dominated delta plain to deltaic channel environments.Whereas,thick beds of sandstone with minor components of carbonaceous material of lime mud and shale with planar cross-bedding is indicating prograding delta-distributaries to delta-font as favorable depositional environments of the C-sand reservoir interval.The reflection patterns associated with the low frequency,low-medium continuity,and medium-weak refection amplitude revealed that thick beds of shales interbedded with glauconitic sandstone,and thin layers of mudstone,siltstone and limestone are present with hummocky clinoform,which suggests the below wave-base deposition of these sediments under low energy environment of mid-shelf marine(pro-delta)settings.3D facies mode was developed using sequential indicator simulation(SIS)method under cellbased facies modeling which showed the actual lithology distribution of the C-sand interval.The model showed that the reservoir sands were deposited in the delta distributaries to delta front settings and thinly laminated finely grained sediments in the form of siltstone,lime mudstone,and few shales are also present in the C-sand interval.The main lithology is coarse sandstone which is 32% in the C-sand reservoir interval of the Sawan gas field.The percentage of other lithologies present within the C-sand are medium sandstone(17.22 %),fine sandstone(14.44 %),muddy siltstone(27.78 %)and shales(8.89 %)respectively.Petrophysical analysis underlined the vertical and lateral extent of the C-sand which is 107.05 m in the Sawan-01 well and were 96.35 m and 94.85 m in the Sawan-07 and Sawan-08 wells which also suggested that the lateral extent of the C-sand is decreasing from NW to SE direction.Which further showed that the C-sand interval has maximum porosity and minimum volume of shale.Gamma ray curve shape analysis revealed that the log trend falls mainly into four categories namely: Irregular trends,funnel-shaped,cylindrical/box car-shaped,and bell-shaped successions.The reservoir interval mainly falls into cylindrical shaped and funnels shaped successions which represent an overall progradation of the C-sand interval.Geological modeling(structure map,thickness map,and depositional model)generated for the study area also showed that the study area is tilting towards to the eastern zone showing an overall progradation of the Lower Goru reservoir sands towards the eastern side.Lithofacies calculation shows that the Csand interval consists of coarse-to-fine grained sandstone particles that were deposited in the fluvial-dominated delta distributaries to delta front environment and while the fine grained sediments consist of siltstone,lime mudstone,and shales.The shales were deposited in the prodelta(transgressive shelf-edge)settings.These results strongly support the results of the integrated facies analysis and developed 3D facies model.These results confirm the reliability of the research work.Furthermore,these results were also confirmed from 3D seismic attribute analysis and were further characterized using the CSSI.Facies logs were created to differentiate the pay sands and wet sands.Besides,cross-plot of the gamma ray and p-impedance highlighted the cutoff values using the shale-shaley sand-sand log created for the target area.The results showed that the cutoff values for the reservoir C-sand lie between the 40 API – 80 API and the impedances cutoff values lies from 7.5e+06 kg/m^3*m/s to1.2e+07 kg/m^3*m/s.The low range of impedance values is associated with fine sand particles while high values of impedances are associates with high coarse reservoir sands.The cross plots showed that more samples are present towards the high impedance values which indicated that the C-sand interval consists of more coarse-to medium sands particles which are associated with gas saturation.Volume and horizon attributes were extracted using the trace gate of target formation.Volume attributes were generated at the time slice of C-sand interval.Relative acoustic impedance attribute demarcated the boundary for the marginal marine and open marine environments.The marked boundary between the delta front and pro-delta environment was overlaid on the volume attributes results of RMS amplitude,envelope,sweetness,instantaneous frequency and phase shift which confirmed that the change in the depositional environment of the C-sand.Moreover,the results of horizon attributes created for the C-sand(X1)and B-sand(Y1)horizons are in agreement with the results of the volume attributes.The results of the half energy showed that the reservoir sands were deposited during regression and shales were deposited during transgression.The results of the instantaneous frequency of the volume attributes and instantaneous phase of horizon attributes showed that bedding of the sands is not continuous due to the presence of the fractures.The presence of fractures was confirmed by using the automatic fault extraction with the help of artificial ant-tracking as an input which confirmed the presence of fractures in the study area.The C-sand had fewer discontinuities,and no fault reached at a depth of the target horizon for the Sawan gas field.The presence of fault close of Sawan-01 well was confirmed by artificial ant-tracking using variance attribute as an input.The time slices of the RMS amplitude,envelope,and sweetness attribute were analyzed at Z = 2188 ms,which highlighted the sweet spots present at the C-sand interval which will be helpful for the future exploitation of the Sawan gas field.Some bright spots were detected below C-sand interval which is present at a depth of B-sand interval within the Sawan gas field.Constrained sparse spike inversion successfully delineated the sand-shale facies and gave the spatial and vertical extent of the C-sand in high resolution.The results also revealed that the low to medium acoustic impedance of the reservoir C-sand is equivalent to the topset of the progradational reflection patterns which suggests that the reservoir sands have good quality.The inverted model was accessed to characterize the reservoir parameter further.Spatial/lithological distribution of porosity within the C-sand interval ranges from 14 % to 18 %.The sand ratio distribution of the facies ranges from 0.8 to 1 which is maximum for the reservoir interval.The GR value ranges from 40 to 80 API for the C-sand interval,and impedances ranges about 1.2+07kg/m^3*m/s.These results are in agreement with the electrofacies analysis,integrated facies analysis,and seismic attribute analysis which was also evident from cross plot results. |
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