Prediction Of Diagenetic Facies Using Well Logs And Its Application

Understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration.As a typical tight sandstone reservoir,the Chang 7 Member of the Upper Triassic Yanchang Formation,Heshui Field,Ordos Basin is an important oil-producing interval.However,varied diagenetic alterations result intense heterogeneity,which can be offset by introducing diagenetic facies.Additionally,distribution of diagenetic facies could be predicted using well logs,which will facilitate reservoir quality prediction.Results of helium porosity and permeability and petrographic assessment from thin sections,X-ray diffraction,scanning electron microscopy and cathodoluminescence analysis,show that Chang 7 Member has encountered varied diagenetic alterations and shows occurrence of various diagenetic minerals,and thus it provides a good opportunity to study the diagenetic alterations and diagenetic heterogeneity of tight sandstones.The sandstones of Chang 7 Member are dominated by fine lithic arkoses,feldspathic litharenites and litharenites.Porosity ranges from0.5%to 13.6%,with an average of 6.8%,and horizontal permeability ranges from0.009×10^-3?m² to 1.818×10^-3?m²,averaged 0.106×10^-3?m².Pore types are mainly remnant intergranular pores,secondary pores and micro-pores,along with partial fractures.The diagenetic alterations comprise dominated mechanical compaction and minor chemical compaction,carbonate cements mainly including ferroan calcite and ferroan dolomite ankerite,quartz cements including quartz overgrowths and microquartz,clay minerals including illite,I/S mixed layers and chlorite,dissolution from unstable minerals such as feldspars and rock fragments.Compactional porosity loss?COPL?shows an average of 28.6%,whereas cementational porosity loss?CEPL?shows an average of 10.7%.On the basis of current data and previous study,the following conclusions have been achieved.Quartz cements are sourced by dissolution of feldspars,clay minerals transformation,volcanic minerals.Precipitation of 5?m³0?m quartz cement needs about 11 Ma⁶7 Ma.Carbonate cements mainly formed during deep burial?below1200 m?.The carbon source were mostly tuff,organic matter decarboxylation,and bacterial fermentation,whereas calcium and magnesium were mainly derived from plagioclase dissolution and clay minerals transformation.Illite were mainly derived from dioctahedral smectite,and partially illitization of kaolinite and feldspar.Chlorite was formed from trioctahedral smectite grain coats,and volcanic minerals and biotite provided iron and magnesium for this conversion.Kaolinite was derived from feldspar dissolution and subsequent reaction with feldspar depleted kaolinite.Organic acid from organic matters and hydrolytic disproportionation reactions of hydrocarbon and water contribute to formation of secondary pores,whereas compactional pore water and meteoric water made much less contribution to secondary pores.Based on the above,the effects of diagenesis on reservoir quality are analyzed.Mechanical compaction plays a dominated role in destroying the reservoir quality.Grain-coating microquartz coating has limited influence on reservoir quality,whereas quartz overgrowths,carbonate cements and pore filling clay minerals destroy reservoir pores.Secondary pores from dissolution of unstable minerals such as feldspar can effectively enhance porosity.In addition,the following ideas are held in this thesis:hydrocarbon emplacement may inhibit cementation of quartz and carbonate,but overpressure shows no influence on porosity preservation.According to the above study,four diagenetic facies are identified,i.e.sandstones with dissolved unstable components facies,sandstones with clay minerals cements facies,sandstones with carbonate cements facies and tightly compacted sandstones/siltstones facies,to divide the heterogeneous reservoir into relatively homogeneous reservoir.By correlating diagenetic facies and well logs,feasibility of diagenetic facies prediction using well logs are performed.Then Crossplots of well logs and principal components are used to construct the prediction models.The model using five-dimensional principal components show much higher accuracy than well logs.Calculation models of porosity and permeability built according diagenetic facies show high precision,which emphasize the crucial role of diagenetic facies in reservoir evaluation.The reservoirs potential index,which is built on the constraint of diagenetic facies,are used to predict distribution of reservoirs with high quality.And the result is consistent with testing and production data.