Controlling Factors Of The Potential Reservoirs Within Low Diagenetic Stage Of Dongying Formation In The In Laoyemiao Oil Field, Eastern Hebei Province

Laoyemiao area of Jidong oilfield is important oil and gas distribution area in the Napu sag of the Bohai bay basin, and Dongying formation is the main production layer of this area, it has been proved that Dongying formation reservoir is of strong heterogeneity by many year’s hydrocarbon exploration and exploitation. Previous study mainly focused on reservoir sedimentology, especially paid attention to the prediction of potential reservoir in hydrocarbon exploration and exploitation; however, the fine study of reservoir on sedimentary fabric analysis, diagenetic fabric analysis and diagenesis has been ignored by previous study, which resulted in several challenges in hydrocarbon exploration and exploitation of this area, such as Dongying formation’s crude oil is of good quality but low outputs, and reservoir physical properties varies laterally. The sources of these problems maybe lie in the ambiguous understanding on controlling factors of Dongying formation reservoir. On diagenetic period, the Dongying formation of Laoyemiao area are in the early period of middle diagenetic stage (A stage) and the late period of early diagenetic stage (B stage), which belong to low diagenetic stage. The reservoirs in low diagenetic stage are very general in China, such as Damintun sag, Qikou sag, Biyang sag and such on, and the reservoir quality of this type is controlled by both sedimentation and diagenesis. In view of this background and condition, this dissertation takes Dongying formation reservoir in low diagenetic stage of Laoyemiao area as the research object, and makes known the controlling factors in low diagenetic stage reservoir by comprehensive analysis of many factors, which gives service to hydrocarbon exploration and exploitation.Based on the geophysical logging data, core data, seismic reflection data and granulometric analysis, the dissertation makes an analysis on Dongying formation reservoir sedimentary system, finds out the sedimentary system types and spatial distribution rules, and depicts the reservoir macro-sedimentary characteristics. By the thin-section observation and identification, this dissertation makes a semi-quantitative and quantitative characterization on sedimentary fabric of Dongying formation reservoir, such as terrigenous clastic grain size, sorting, clast roundness, contact relationship between clastic grains and so on, which depict the reservoir micro-sedimentary characteristics. The dissertation divides the reservoir diagenesis stages by vitrinite reflection and montmorillonite’s percentage content in montmorillonite and illite mixed layer, and makes an analysis on reservoir diagenesis types and characteristics by means of many testing technology, such as polarization microscope, scanning electron microscope and cathodoluminescence micrography. On the basis of spatial distribution on single-well diagenesis type, the author makes combined well diagenesis analysis, and makes known the time and spatial distribution rules of diagenetic facies, and makes a quantitative study on reservoir diagenetic fabric analysis by the studies of cement type and content, together with clay minerals assemblage and content. Based on the microscopic-analysis on casting thin sections and reservoir physical properties test, we can find out the reservoir pore texture and physical properties characteristics. On the basis of this, the author discusses the influence of reservoir physical properties caused by sedimentation and diagenesis individually, which helps judge the main controlling factors on reservoir quality. The main understanding and conclusions have been obtained as follows:(1) Established the recognition mark of Dongying formation’s sedimentary system in Laoyemiao area, recovered the sedimentary system tracts, and summarized the reservoir sedimentary model, and on the basis of this, the author made a quantitative to semi-quantitative study on reservoir sedimentary fabric.By the means of typical well core observation, single-well vertical sequence, granulometric analysis, seismic facies and seismic attribute analysis, the author recognized three main sedimentary systems in the interior of paleogene Dongying formation in the Laoyemiao area. They are fan delta sedimentary system, lacustrine sedimentary system and turbidite sedimentary system. The fan delta sedimentary system mainly distributed in the north part of the study area, it progressed from north to south in general, and it spread widely and had an important influence to the study area; the lacustrine sedimentary system mainly distributed in the south part of the study area, it also spread relatively widely; the turbidite sedimentary system mainly distributed in the southwest part of the study area, it was characterized by a small range of extension. In general, the stratigraphy was presented with retrogradation to progradation sedimentary sequence from Lower part of Member-3of Dongying formation to Member-1of Dongying formation, and the Member-2of Dongying formation was affected obviously by the lacustrine sedimentary system. As a whole, the Dongying formation reservoir is of moderately sorted and poorly roundness, the sorting of reservoir is getting a little better from Lower part of Member-3of Dongying formation to Member-1of Dongying formation, and the contact relationship between grains in a dominant position transformed from point-line contacts to point contacts. The terrigenous elastics of Dongying formation reservoir are consisted of quartz, alkali feldspar and rock debris, together with a small quantity of plagioclases, the content of these terrigenous clastics in different members of Dongying formation are basically stable, which maybe revealed that Dongying formation had a relatively stable source supply during the sedimentary phase.(2) Ascertained the reservoir diagenetic stages of Dongying formation in the Laoyemiao area, summarized the reservoir diagenesis types, diagenetic facies in time and spatial distribution rules, and made a quantitative study on reservoir diagenetic fabrics.Based on clay mineral and vitrinite reflection analysis of several drilling cores from study area, the author divided the reservoir diagenetic stages of the study area into two stages, the first stage is in the late period of early diagenetic stage (B stage) and the second stage is in the early period of middle diagenetic stage (A stage), and the late period of early diagenetic stage is corresponding to the first member of Dongying formation and the upper part in second member of Dongying formation, and the burial depth is less than3500m, whereas the early period of middle diagenetic stage is corresponding to the third member of Dongying formation and the lower part in second member of Dongying formation, and the burial depth is more than3500m. With the help of thin-section observation, scanning electron microscope and cathodoluminescence micrography, the author makes a conclusion that the reservoir diagenesis types are mainly consisted of compaction, calcite cementation, dolomite cementation, clay minerals cementation, siliceous cementation, feldspar dissolution, detrital minerals dissolution, and calcite dissolution. The single-well diagenetic facies study indicated that the diagenesis types of the third member of Dongying formation and the lower part in second member of Dongying formation (the early period of middle diagenetic stage) are characterized with strong compaction, siliceous cementation, calcite cementation, little detrital minerals dissolution and calcite dissolution; whereas the diagenesis types of the first member of Dongying formation and the upper part in second member of Dongying formation (the late period of early diagenetic stage) are characterized with weak compaction, detrital minerals dissolution, calcite dissolution, little clay mineral cementation and dolomite cementation. The cement contents and types of Dongying formation reservoir of the study area varies regularly, for the same horizon, the distributary channels and under-water distributary channels sandstone or coarse sandstone has low content of calcite cement, for the same genetic facies, the upper part has low content of calcite cement and siliceous cement, whereas has relatively higher content of dolomite cement, authigenic clay minerals and siderite. For the same horizon, the distributary channels and under-water distributary channels sandstone or coarse sandstone has relatively high content of kaolinite, for the same genetic facies, the upper part has high content of kaolinite and low content of chlorite and illite, which means that the content of kaolinite increases from the lower part to upper part, whereas the content of chlorite and illite decreases from the lower part to upper part.(3) Ascertained the pore structure characteristics of Dongying formation reservoir in the study area, and summarized reservoir physical properties of different genetic facies and different rock types.The reservoir pore types of Dongying formation of the study area are mainly intergranular pore, intragranular pore, moldic pore, intercrystalline pore and microfissure. These five pore types distribute in Lower part of Member-3of Dongying formation, upper part of Member-3of Dongying formation, Member-2of Dongying formation and Member-1of Dongying formation. The reservoir parameters such as porosity, permeability, the average pore throat radius and the maximal pore radius varies regularly. For the same genetic facies or lithology, these reservoir parameters increase gradually in whole from Lower part of Member-3of Dongying formation to Member-1of Dongying formation. For the same horizon, these parameters in distributary channels and under-water distributary channels or coarse sandstone reservoir are relatively larger. Specifically, for the same horizon, the reservoir physical properties in distributary channels and under-water distributary channels sandstone are better, whereas for the same genetic facies, the reservoir physical properties are better in the upper part; for the same horizon, middle-coarse sandstone’s reservoir physical properties are better, whereas for the same lithology, the reservoir physical properties are also better in the upper part. From the reservoir physical properties section distribution, the author makes a conclusion that reservoir physical properties are both controlled by horizon and depth in vertical direction, the reservoir physical properties of Member-1of Dongying formation and upper part of Member-2of Dongying formation are evidently better than Member-3of Dongying formation and lower part of Member-2of Dongying formation, and the reservoir physical properties are getting worse toward southeast in plane.(4) Discussed the influence of reservoir physical properties caused by sedimentation and diagenesis individually, and summarized the controlling factors on good-quality reservoir systematically.For the sedimentary fabric, the Dongying formation reservoir rock structure has an evident influence on reservoir physical properties, the reservoir physical properties are evident better when the clastic grain sizes are of relatively coarse and moderately well sorted as well as subangular to subrounded roundness or subrounded roundness, together with high content of clastic grain and low content of matrix. This type of reservoir with good-quality are commonly developed in these genetic facies, such as distributary channels of fan delta plain and under-water distributary channels of fan delta front. For the diagenetic fabric, the reservoir physical properties are affected evidently by both diagenesis types and diagenetic stages, with the increasing of cement content (especially the calcareous cements), the reservoir physical properties gets worse evidently, whereas with the increasing of kaolinite content, the reservoir physical properties gets better evidently. In the late period of early diagenetic stage, the clay minerals and feldspar are in a state of dissolution, and then form many intergranular pore and moldic pore, which enhance the reservoir physical properties largely, conversely, in the early period of middle diagenetic stage, calcite cementation and compaction are extremely evident in reservoir, and the primary porosity or secondary porosity decreases or has been jammed by these destructive diagenesis, which result in reservoir physical properties getting worse evidently. From the aspect of diagenesis and porosity evolution, the diagenesis which has the evident influence on reservoir primary porosity are compaction and calcite cementation, they directly result in the decrease of primary porosity, and feldspar dissolution and clastic minerals dissolution are the key geological factors to the development of secondary porosity in the study area, with the company of them are usually the formation of the clay minerals such as kaolinite. In general, the potential reservoir’s development is controlled by both sedimentation and diagenesis, and the sedimentation affects the reservoir’s material composition and fabrics, which determine the pathway and degree of diagenesis, and then affect the reservoir physical properties.(5) On the basis of comprehensive study of sedimentation, diagenesis and reservoir physical properties in the study area, the dissertation makes a classified evaluation on the potential reservoir in the study area, and points out the favorable zone of hydrocarbon exploration. The Dongying formation reservoir is in the low diagenetic stage, and the reservoir quality is controlled by both sedimentation and diagenesis. The reservoir physical properties of fan delta plain are best from the aspect of sedimentology, and then the under-water distributary channels and proximal sandy mouth bars of fan delta front. From the aspect of diagenesis, the Member-1of Dongying formation is in the late period of early diagenetic stage, the diagenesis is mainly consisted of feldspar dissolution and clastic minerals dissolution, and the reservoir physical properties are the best; the Member-2and upper part of Member-3of Dongying formation are in the late period of early diagenetic stage or the early period of middle diagenetic stage, the diagenesis is mainly consisted of clastic minerals dissolution and little feldspar dissolution together with clay minerals cementation, and the reservoir physical properties are relatively better; the lower part of Member-3of Dongying formation is in the early period of middle diagenetic stage, the diagenesis is mainly consisted of calcite cementation, dolomite cementation, clay minerals cementation and little clastic minerals dissolution, and the reservoir physical properties are relatively worse. On the analysis of known reservoir physical properties data, the dissertation points out that the Member-1of Dongying formation reservoir is mainly the reservoir of Ⅱ-Ⅲ type; the Member-2and upper part of Member-3of Dongying formation is mainly the reservoir of Ⅲ-Ⅳtype; and the lower part of Member-3of Dongying formation is mainly the reservoir of Ⅳ-Ⅴ type. On the basis of this, the dissertation makes a prediction on the potential reservoir in the study area, and points out that the best potential reservoir(Ⅱ-Ⅲtype) in the study area is distributed in the Member-1of Dongying formation, and distributed in the north part of the study area in spatial; the better potential reservoir(III-IVtype) in the study area is distributed in the Member-2and upper part of Member-3of Dongying formation, and distributed in the north and south part of the study area in spatial; the worse potential reservoir(IV-VItype) in the study area is distributed in the lower part of Member-3of Dongying formation, and distributed in the south part of the study area in spatial. Based on the previous study above, combining with structural condition and petroleum geology analysis, the dissertation makes a prediction on the favorable zone of hydrocarbon exploration, and points out that main body of anticline in the north part of Laoyemiao area develops fault-nose structural oil and gas reservoir, structural-lithology oil and gas reservoir and fault-blocked lithology oil and gas reservoir, and the reservoir sandstone are mainly distributary channels sandstones in fan delta plain and under-water distributary channels sandstones in fan delta front, this area is of high density of drilling and of high degree of hydrocarbon exploration, and it is the best favorable zone(Ⅰ type); the faulted structure zone in the south of Laoyemiao area mainly develops fault-blocked lithology oil and gas reservoir and lithology-structural oil and gas reservoir, and the reservoir sandstone are mainly under-water distributary channels sandstones and mouth bars sandstones in fan delta front, this area is the secondary favorable zone(ⅡA type); the tail part and both flanks of anticline in the north part of Laoyemiao area mainly develops fault-blocked lithology oil and gas reservoir, volcanic rock-blocked lithology oil and gas reservoir, this area is the third favorable zone(ⅡB type); the downthrown side of Xi’nanzhuang fault develops skirt-like structural oil and gas reservoir, the reservoir sandstone is mainly turtbidite sand body, the reservoir is of poor quantity, and this area is the last favorable zone(Ⅲ type).The dissertation has made the following major innovative achievements through the above studies:①Take the Dongying formation reservoir in low diagenetic stage in the Laoyemiao area of Jidong oilfield for example, the dissertation systematically illustrates the influence of reservoir quality caused by both sedimentation and diagenesis, which makes a foundation for distinguishing the main controlling factor on reservoir quality, and the researches provide an important value on the assessment of potential reservoir of good quality of Dongying formation in the Laoyemiao area, and they are also helpful to predict the favorable zones for hydrocarbon exploration.②By the systematical study on reservoir sedimentology, reservoir diagenesis and reservoir physical properties, the dissertation deeply reveals the reservoir characteristics and heterogeneity in low diagenetic stage, as well as points out the controlling factor in potential reservoir of good quality. The dissertation puts sedimentation and diagenesis and reservoir physical properties as a whole to study, and the research method can be used for reference to the same type reservoir’s heterogeneity research and genesis of good-quality reservoir.