Application of seismic sedimentology in characterization of fluvial-deltaic reservoirs in Xihu sag, East China Sea shelf basin

The fluvial-deltaic reservoirs of the Oligocene Huagang Formation in the Xihu sag of the East China Sea shelf basin reflect rapid lateral change in sedimentary facies and poor morphology of conventional slice attributes,which bring difficulties to the reservoir prediction for subsequent exploration and development of lithologic reservoirs.The traditional seismic sedimentology technology is optimized by applying the characteristic technologies such as frequency-boosting interpretation,inversion-conventional–90°phase shift joint construction of seismic lithologic bodies,nonlinear slices,paleogeomorphology restoration,and multi-attribute fusion,to obtain typical slice attributes,which are conducive to geological form description and sedimentary interpretation.The Huagang Formation developed three types of sedimentary bodies:braided river,meandering river and shallow water delta,and the vertical sedimentary evolution was controlled by the mid-term base-level cycle and paleogeomorphology.In the early–middle stage of the mid-term base-level ascending cycle,braided channel deposits were dominant,and vertical superimposed sand bodies were developed.In the late stage of the ascending half-cycle and the early stage of the descending half-cycle,meandering river deposits were dominant,and isolated sand bodies were developed.In the middle–late stage of the descending half-cycle,shallow-water delta deposits were dominant,and migratory medium–thick sand bodies were developed.Restricted paleogeomorphology controlled the sand body distribution,while non-restricted paleogeomorphology had little effect on the sand body distribution.Based on reservoir characterization,the fault sealing type and reservoir updip pinch-out type structural lithological traps are proposed as the main directions for future exploration and development in the Xihu sag.

Study of the Seepage Mechanism in Thick Heterogeneous Gas Reservoirs

The seepage mechanism plays a crucial role in low-permeability gas reservoirs.Compared with conventional gas reservoirs,low-permeability sandstone gas reservoirs are characterized by low porosity,low permeability,strong heterogeneity,and high water saturation.Moreover,their percolation mechanisms are more complex.The present work describes a series of experiments conducted considering low-permeability sandstone cores under pressuredepletion conditions(from the Xihu Depression in the East China Sea Basin).It is shown that the threshold pressure gradient of a low-permeability gas reservoir in thick layers is positively correlated with water saturation and negatively correlated with permeability and porosity.The reservoir stress sensitivity is related to permeability and rock composition.Stress sensitivity is generally low when permeability is high or in the early stage of gas reservoir development.It is also shown that in sand conglomerates,especially the more sparsely filled parts,the interstitial materials among the conglomerates can be rapidly dislodged from the skeleton particles under stress.This material can therefore disperse,migrate,and block the pore throat producing serious,stress-sensitive damage.

Influence of Wellbore Trajectory on Pressure Drop and Fluid Discharge

An experimental analysis has been conducted to study the process of fluid accumulation for different borehole trajectories.More specifically,five heel angles have been experimentally realized to simulate the borehole trajectory of the sloping section of the formation.The fluid-carrying capacity,pressure drop and fluid discharge volatility have been investigated for these conditions and,accordingly,the relationship between heel angle and wellbore pressure drop fluid-carrying capacity has been determined.The results show that while the reasonable roll angle can increase the pressure loss in the wellbore,it is beneficial to drainage.In terms of pressure loss and liquid-carrying capacity,when the heeling angle is 50°,the latter is increased while the former becomes very high,which indicates that when drilling and completing wells on site,a 50°roll angle should be avoided.It is found that the main reason for the increase of the total pressure drop in the wellbore is the increase of the local pressure loss in the inclined section.From the perspective of drainage stability,when there is heeling in the inclined section of the horizontal well,the fluctuation of the wellbore drainage tends to be enhanced.Through the comparison of the Beggs-Brill(B-B)and Mukherjee-Brill liquid holdup methods,it is found that B-B method better predicts liquid holdup.A new method for calculating the pressure drop in the inclined section in the presence of lateral inclination is obtained by taking into account the pressure drop in the curved section.Through comparison with experimental data,it is found that the error is within 20%,and the prediction accuracy is high.

Diagenesis and Diagenetic Evolution of Deltaic and Neritic Gas-Bearing Sandstones in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin: Implications for Depositional Environments and Sequence Stratigraphy Controls

Gas-bearing deposits in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin consist of shoreface sandstones of the highstand systems tract(HST) and transgressive systems tract(TST), and deltaic sandstones of the lowstand systems tract(LST) and falling stage systems tract(FSST).Detailed petrographic observations suggest that the diagenetic features and related evolution of these deposits cannot be simply characterized and demonstrated in the depth domain.However, the occurrence of diagenetic minerals systematically depends on the studied interval within the HST, TST, LST, and FSST; therefore, diagenesis in this region can be better constrained when studied in the context of the depositional environments and sequence stratigraphic framework.The eogenetic processes in such settings include:(1) microcrystalline siderite precipitated as concretions in almost all environments and systems tracts, which inhibited further mechanical compaction;(2) grain dissolution and kaolinitization occurred in shoreface HST sandstones and deltaic LST and FSST sandstones;(3) glaucony was locally observed, which did not clearly reflect the controls of facies or sequence stratigraphy; and(4) cementation by pyrite aggregates occurred in the shoreface HST sandstones and deltaic LST sandstones.The mesogenetic diagenesis includes:(1) partial conversion of kaolinite into dickite in deltaic LST sandstones, and minor chlorite cementation in deltaic FSST sandstones;(2) transformation of kaolinite into illite and quartz cementation in deltaic LST and FSST sandstones;(3) frequent precipitation of ankerite and ferroan calcite in shoreface TST sandstones and early HST sandstones, forming baffles and barriers for fluid flow, with common calcite in shoreface HST sandstones as a late diagenetic cement; and(4) formation of dawsonite in the deltaic LST and FSST sandstones, which is interpreted to be a product of the invasion of a CO2-rich fluid, and acts as a good indicator of CO2-bearing reservoirs.This study has thus constructed a reliable conceptual model to describe the spatial and temporal distribution of diagenetic alterations.The results may provide an entirely new conceptual framework and methodology for successful gas exploration in the continental margins of offshore China, thus allowing us to predict and unravel the distribution and quality evolution of clastic reservoirs at a more detailed and reliable scale.

Facies Architecture Model of the Shimentan Formation Pyroclastic Rocks in the Block-T Units, Xihu Sag, East China Sea Basin, and its Exploration Significance

A gas field consisting of volcanic reservoir rocks was discovered in the block-T units of the Xihu Sag,East China Sea Basin.The lithology of the volcanic rocks is dominated by tuff and reworked tuff.The lithofacies are dominated by base surge deposits of explosive facies.As the architecture model of volcanic facies is still uncertain,it has restricted the exploration and development of mineral resources in this area.Using core and cuttings data,the lithology,lithofacies,geochemistry as well as grain size characteristics of volcanic rocks were analyzed.Based on these analyses,the volcanic rocks in the well section are divided into three eruptive stages.The transport direction of each volcanic eruption is analyzed using crystal fragment size analysis.The facies architecture of the block-T units was established based on the reconstruction results of paleo-geomorphology.The results show that the drilling reveals proximal facies(PF)and distal facies(DF)of the volcanic edifices.However,the crater-near crater facies(CNCF)are not revealed.Compared with the reservoirs of the Songliao Basin,it is shown that the volcanic rocks in the Xihu Sag have good exploration potential;a favorable target area is the CNCF near the contemporaneous fault.

Forming Condition and Geology Prediction Techniques of Deep Clastic Reservoirs

1 Introduction As new exploration domain for oil and gas,reservoirs with low porosity and low permeability have become a hotspot in recent years(Li Daopin,1997).With the improvement of technology,low porosity and

Study of the Sensitive Properties of Marine Gas Hydrate Based on the Prestack Elastic Inversion

Using a bottom simulating reflector(BSR)on a seismic profile to identify marine gas hydrate is a traditional seismic exploration method.However,owing to the abundance differences between the gas hydrate and free gas in different regions,the BSR may be unremarkable on the seismic profile and invisible in certain cases.With the improvement of exploration precision,difficulty arises in meeting the requirements of distinguishing the abundance differences in the gas hydrate based on BSR.Hence,we studied other sensitive attributes to ascertain the existence of gas hydrate and its abundance variations,eventually improving the success rate of drilling and productivity.In this paper,we analyzed the contradiction between the seismic profile data and drilling sampling data from the Blake Ridge.We extracted different attributes and performed multi-parameter constraint analysis based on the prestack elastic wave impedance inversion.Then,we compared the analysis results with the drilling sampling data.Eventually,we determined five sensitive attributes that can better indicate the existence of gas hydrate and its abundance variations.This method overcomes the limitations of recognizing the gas hydrate methods based on BSR or single inversion attribute.Moreover,the conclusions can notably improve the identification accuracy of marine gas hydrate and provide excellent reference significance for the recognition of marine gas hydrate.Notably,the different geological features of reservoirs feature different sensitivities to the prestacking attributes when using the prestack elastic inversion in different areas.

Diagenesis and diagenetic stages prediction of Ed2 reservoir in the west of Bozhong sag

Diagenesis of clastic rock and hydrocarbon generating capacity are closely related with diagenetic stages.Based on diagenetic evolution,reservoir diagenetic stages prediction method is proposed through making a contrastive study of simulation results and test results using core measurement data.The essence of this method is illustrated and its effectiveness is demonstrated using Ed2 clastic sandstones in the west of Bozhong sag,Bohai Bay Basin,China.Measurement of paleo temperature(T),vitrinite reflectivity(Ro%)and the proportion of smectite in illite/smectite interstratified minerals(I/S-S%)indicate that there are there types of diagenetic stages in the study area,including early diagenetic stage B,middle diagenetic stage A1 and middle diagenetic stage A2.When only considering T index for diagenetic stages prediction,the simulation results are more conservative than measured data with some situations,and the simulation result using Ro%for diagenetic prediction is less than measured data in some wells.When I/S-S%is used,the two situations above both exist.Because depth,temperature,time duration,pressure and some chemical variables can change synchronously or asynchronously,it is difficult to attribute with certainty the parameters that affect the apparent diagenetic stages evolution series.Diagenetic stage variations can be significantly different in different parts of one sedimentary basin.A synthetic indexes method considering T,Ro%,I/S-S%to predict its distribution,and the simulation result has proved that the reliability of the model has greatly improved.