Characterization Of Low Permeable To Tight Es3 Lacustrine Turbidites Reservoir From Dongying Depresion,Bohai Bay Basin,East China

Deep-water,marine turbidite sandstones are known to form moderate to excellent petroleum reservoirs.The architecture and quality of lacustrine turbidites that act as petroleum reservoirs are less documented.Reservoir architecture and multiscale(macroscopic,mesoscopic,microscopic)heterogeneity in turbidites represent serious challenges to production performance.Macroscopic heterogeneity deals with distribution of impervious silt or mud units in between the stacked architectural elements.While distribution of lithofacies and formation of composite flow units within beds represent mesoscopic heterogeneity.Microscale heterogeneity represent the complexities in texture and fabric of rock which are characterized by the distribution of microstructures such primary porosity,secondary porosity,microporosity,clay and cement within the pore space.The inherited pore structure in natural rocks affect the processes that directly control fluid flow.These rocks exhibit irregular,tortuous,nonhomogeneous pore structure,especially complexity worsen in turbidite rocks where wide variety of deposition and diagenetic process govern rock fabric.Characterization and quantification of these structures in reservoir rocks is gaining significant importance for oil and gas recovery.Rocks,can be considered as an arbitrary two-phase system;pore space and solid space.Dealing with pore phase is critical,because it is made-up of multi-scale,complex and irregular microstructures.Fractal geometry provides alternative means to study and analyze textural properties of rocks.It can successfully characterize chaotic and heterogeneous structures at multi scales.Scaling porosity of sedimentary rocks from the scale of measurement to the scale of interest is still an open problem.Here,we report on the turbidites of the middle third member(Es3m)of the Eocene Shahejie Formation,where prevailing reservoir heterogeneity at multiple scale is main issue in petroleum exploration,production and remaining oil recovery in this reservoir.An integrated approach is adopted to characterize heterogeneity at three scales;(1)architecture element and sequence stratigraphic scale,lithofacies scale and pore scale.Seismic data,wireline log,and well core is integrated to delineate the architectural,sequence stratigraphic framework.Core observations and analysis are used to interpret depositional mechanism,gravity flow types and lithofacies.A petrographic based approach is adopted to study microscopic heterogeneity using optical microscope,scanning electron microscope(SEM),X-ray computed tomography(micro-CT),mercury Injection Capillary Pressure(MICP),routine core analyses and X-ray diffraction(XRD)analyses.The state of the art and advanced digital rock analysis(DRA)techniques is used to understand heterogeneity and fluid flow at pore scale.Image segmentation and pore network modeling are crucial stages in micro-CT based pore scale rock modeling.The success of the pore network model(PNM)to predict petrophysical properties relies on image segmentation,image resolution and most importantly nature of rock(homogenous,complex or microporous).The pore network modeling has experienced extensive research and development during last decade,however the application of these models to a variety of naturally heterogenous reservoir rock is still a challenge.Es3m is lobe dominated turbidite fan system with channelized base and composed of channels,levees,overbank splays,lobes and lobe fringes architectural elements.Detailed study and following recent research in deepwater fan systems,these deposits are characterized into four–fold hierarchy arrangements from bed through to element,channel/lobe and channel/lobe complex.The Es3 m member is interpreted as a sequence set that is composed of four composite sequences: CS1,CS2,CS3 and C S4.A total of forty-five sequences are identified within these four composite sequences.Throughout the hierarchy,the sands layers are separated by mud layers,similarly in sequence stratigraphy framework,lowstand sands are capped by transgressive or hi ghstand silt or mud layers.Amalgamated sand beds are usually present in channels and proximal areas of lobes which indicate high flow energy.Hybrid event beds are present towards distal area of fan,because of portioning,increased concentration and entrainment of substrate mud with flow run-out downslope.The bounding units in architectural framework and transgressive or highstand caps in sequence stratigraphic framework at all hierarchical scales offer resistance to fluid flow and produce compartmentalized reservoir.The variegated lithofacies assemblage in hybrids beds produces highly composite flow units.On the other hand,amalgamation produces connected clean,massive and high-quality reservoir sands facies beds.The dominant kaolinization of feldspar and mobilization of kaolinite with fluid flow enhanced the quality of the reservoir by producing secondary enlarged pores CS1 and CS2.In this study,four samples from a representative reservoir facies are chosen to characterize the pore scale heterogeneity and petrophysical properties of Es3 m reservoir using high resolution micro-CT imaging.The image segmentation analysis from microCT images show that 5-6 % micropourous regions are present in kaolinite rich sandstone(E3 and E4),while 1.7-1.8 % are present in illite rich sandstone(E1 and E2).The pore system percolate without micropores in E1 and E2 while it does not percolate without micropores in E3 and E4.In E1 and E2,total MICP porosity is equal to the volume percent of macropores determined from micro-CT images,which indicate that the macropores are well connected and microspores do not play any role in non-wetting fluid(mercury)displacement process.Whereas in E3 and E4 sandstones,the volume percent of micropores is far less(almost 50 %)than the total MICP porosity which means that almost half of the pore space was not detected by the micro-CT scan.PNM behaved well in E1 and E2 where better agreement exists in PNM and MICP measurements.While E3 and E4 exhibit multiscale pore space which cannot be addressed with single scale PNM method,a multiscale approach is needed to characterize such complex rocks.2D SEM and 3D micro-CT image were used to to calculate fractal porosity.One sample from Es3 turbidite sandstone and for comparison with other rock type a carbonate samples are chosen.The given equation for fractal porosity is used to scale porosity from image scale to core plug scale.A systematic imaging plan is deployed to capture rock properties of a carbonate and a sandstone sample,which are sensitive to fractal nature of these rocks.Image analysis record completely different pore spectrum and corresponding fractal dimensions in these two rocks.Median and volume weighted average of pore radius and fractal dimensions derived from image analysis are used as inputs fractal porosity equation.The equation yielded best match with porosity measured on core plug scale on a resolution of 2.5 ?m/voxel/voxel in sandstone and 4 ?m/voxel/voxel resolutions in carbonate sample in 3D micro-CT images.Whereas 2D SEM images also provided hopeful results in sandstone sample.The given fractal porosity equation provides good estimation even in those rocks where conventional imaging techniques cannot captured full pore spectrum.This study represents a significant insight into the reservoir architecture and heterogeneity of lacustrine turbidites;these understandings can be applied to exploit primary and secondary production from these fields.The application of existing micro-CT based petrographic characterization methodology to naturally complex Es3 m petroleum reservoir provided in-depth insights about the heterogeneity and behavior of rock towards fluid flow.Outcomes from fractal porosity equation showed promising results to scale porosity,indeed it shows great potential to estimate porosity at multiple scales.It offers an alternative way to measure porosity when there are no routine core measurements are available.