Numerical Simulation of Diagenetic Stage in Sandstone Reservoir of Huagang Formation in Xihu Sag

To predict distribution of diagenetic stages of Xihu Sag Huagang Formation sandstone reservoir and secondary pore development zone, and research original pore reservation and secondary pore increase and decrease in reservoir evolution process, combining traditional diagenesis research and numerical simulation technique, and based on interaction model and effect model, this paper establishes a aggregative model on numerical simulation of diagenetic stages. In diagenesis simulation process, based on process response principle, and restricted by current diagenetic stage distribution and type, diagenetic stage distribution and type of reservoir during geological historical evolution period is back stripped through diagenetic temperature. Taking single well diagenetic stage data as constraint condition, and combining with research area burial history, ground temperature history, and diagenetic stage division basis, and scheme, plane distribution of diagenetic stages of the whole research area during each geological historical period is finally obtained.


Introduction
The evolution of reservoirs pore is mainly controlled by diagenesis and sedimentary facies, where sedimentary facies affects pore evolution at shallow and deep buried depth, respectively [1][2][3]. As an essential process for the generation and development of all kinds of reservoirs, diagenesis fundamentally determines the physical property of all reservoirs, which is especially true for low-porous and lowpermeable reservoir [4][5][6].
Clastic rock diagenetic stage represents different geological historical evolution stages experienced by clastic sediment under all kinds of diagenesis transformation after deposition to the period before metamorphism [7]. Whether hydrocarbon source rock generates hydrocarbon is closely concerned with diagenetic stages, and post mature hydrocarbon source rock (Ro>1.3%) is generally unbeneficial to oil gas generation, and therefore, research on diagenetic stages of research area contributes to beneficial oil-gas exploration area finding [8,9]. Diagenetic stage of clastic sediment in burial stage is closely concerned with oil and gas accumulation property of clastic pore development and evolution to reservoir stratum.
Diagenesis and pore evolution not only affect pore formation, increase and decrease of oil and gas reservoir stratum, but also affect primary pore preservation, secondary pore distribution and pore connection and penetration property. Therefore, diagenesis and pore evolution have basic effect on quality of reservoir, and in research on diagenesis of reservoir, it is necessary to divide diagenesis stage and pore evolution stage of research area.

Geology
Xihu Sag sits in the shelf basin in the northeast of East Sea, distributed in direction of NNE. It is connected with Hupijiao Reef, Haijiao Reef, and Yushan Uplift in west, and adjacent to Diaoyu Island magmatic rock zone in east, and respectively transits to Fujiang depression and Jilong depression through a high saddle in north and south ( Figure  1). Xihu Sag is generally a structural pattern of "two depressions and one uplift", and has structural feature of "east-west zoning and south-north blocking".
According to Cenozoic structural pattern, depositional feature, fault development and the characteristics of oil gas etc., Xihu Sag is classified into 3 secondary structural units, i.e. west gentle slope (slope) belt, central depressioninverted structure belt and east continental slope fault-uplift belt [10,11], from west to east, where, central depressioninverted structure belt includes east sub-depression, central inverted structure belt and west sub-depression from east to west, and central inverted structure belt can be further divided into Jiaxing structure belt, Ningbo structure belt, Huangyan structure belt, Tiantai structure belt and Jilong structure belt from north to south.
From the bottom to the top, the Xihu Sag develops the formation of Eocene Bajiaoting and Pinghu, Oligocene Huagang, Miocene Longjing, Yuquan, and Liulang, Pliocene Santan formation and Pleistocene the East China Sea group etc. Oligocene Huagang formation is one of the main target formations at present, and can be divided into Hua upper section (H1-H4) and Hua lower section (H5-H8) from the top to the bottom.

Diagenetic Stage Simulation Principle
Process response principle is basic scientific principle. Response characteristic depends on effect process. The reservoir is common result of sedimentary, diagenetic and tectonic action in geological history. Considering all kinds of geologic process to reservoir during different periods, especially the condition difference in geologic process at different periods, and effect result of these differential geological conditions, good means can be provided for prediction to reservoir at high diagenetic stage [12].
In diagenesis simulation process, restricted by current diagenetic stage distribution and type, diagenetic stage distribution and type of reservoir during geological historical evolution period is back stripped through diagenetic temperature. Taking single well diagenetic stage data as constraint condition, and combining with research area burial history, ground temperature history, and diagenetic stage division basis, and scheme, plane distribution diagram of diagenetic stages of the whole research area can be obtained [13,14].

Diagenetic Stage Division Basis
Main basis of current clastic rock diagenetic stage division and research of China is Specification for Oil and Gas Industry Standard Clastic Rock Diagenetic Stage Division of the People's Republic of China, and main parameters on diagenetic stage division in the specification includes: (1) paleogeo temperature, (2) maturity of organic matter, (3) clay mineral evolution, (4) distribution and formation sequence of authigenic mineral in sandstone, (5) rock structure, construction features and pore type etc.
Clay mineral is mineral widely existing in clastic rocks, and it is generally considered that in diagenesis research, clay mineral combination and clay mineral mixed-layer ratio size, I/S mixed layer clay mineral evolution and organic matter evolution maturity are main basis on diagenetic stage division, and make diagenesis research developed from qualitative research to quantitative research. According to new diagenetic stage division standard (No. SY/T5477-2003) in oil and gas industry of the People's Republic of China (Table 1), diagenetic stages are classified according to percentage composition of smectite in clay mineral I/S and vitrinite reflectance (Ro) characteristics, with thermal maturity of organic matter and clay mineral adopted as main indexes.

Vitrinite Reflectance Analysis
Seen from Ro test data of partial wells of research area, Ro value of most wells is 0.60%-0.85% (Table 2), and with increase of burial depth, Ro value presents increase tendency, and organic matter maturity is at low mature -mature stage, being at middle diagenetic stage A.

Analysis on Relative Content of Clay Minerals of Reservoir
According to diagenetic stage division standard, in illite / smectite mixed-layer, content of smectite (%S) ＜15% is the sign for entry into middle diagenetic stage B ( Figure 2). Diagenetic evolution experienced by Huagang formation reservoir of each tectonic region of Xihu Sag does not exceed scope of middle diagenetic stage, and is dominated by middle diagenetic stage A.

Analysis on Maximum Temperature of Thermolysis
According to diagenetic stage division standard, when maximum temperatures of thermolysis (Tmax) in thermal evolution process of organic matter is greater than 490°C, rock starts to enter into late diagenetic stage.
Seen from test data, maximum temperatures of thermolysis (Tmax) displayed in reservoir rock thermolysis analysis data of Xihu Sag is between 431°C-479°C (Table 3). According to diagenetic stage division standard of oil and gas industry of China in 2003, it is determined that Huagang formation reservoir sandstone of each block of Xihu Sag is totally at the stage of middle diagenetic, and no reference about the late diagenetic stage evidence is found. Through comprehensive analysis to evolution characteristics and diagenetic evolution sequence of hydrocarbon source rock of each block of Xihu Sag, diagenetic change of Huagang formation is divided into 3 stages and 4 stage times, i.e. syndiagenetic stage, early diagenetic stage A and B, and middle diagenetic stage A and B.
Although current diagenetic stage of north central part and south central part of central inverted structure belt deviates from diagenetic environment in a certain degree, there is no big difference in overall evolution process [15]. Therefore, combining with predecessor's research, it is considered that diagenetic sequence and diagenetic environment of Xihu Sag Huagang formation reservoir sandstone in overall evolution process successively experience [16]: alkalescence of early diagenetic stage A →weak acidity of early diagenetic stage B →acidity of middle diagenetic stage A-acid-base transition belt →alkalinity of middle diagenetic stage B.

Single Well Diagenetic Stage
Based on basic cognition to overall evolution route of diagenetic sequence and diagenetic environment of reservoir sandstone of research area, vertical division research aimed at diagenetic sequence and diagenetic environment of typical drilling of each main tectonic region within depression is further developed. Select well NB27-5-2 of research area located in west sub-depression for illustration ( Figure 3). Upper part of Liulang formation of well NB27-5-2 of west sub-depression and shallow layer (＜1400m), alkalescence diagenetic environment, according to burial-thermal history: Ro＜0.35%, early rimmed chlorite, at early diagenetic stage A.
Upper part of Yuquan formation to lower part of Liulang formation (1400m-2300m), faintly acid diagenetic environment, according to burial-thermal history: 0.35%＜ Ro ＜ 0.5%, stage 1 acidity dissolution, early calcite cementing starting, point contact, quartz level-I secondary enlargement (autogenic kaolinite may begin to be developed), at early diagenetic stage B.
Vertical evolutionary analysis on diagenetic stage division and diagenetic environment of typical well at each tectonic division of research area shows: middle diagenetic stage A1 has relatively strong acidity diagenetic environment, and Huagang formation at the diagenetic stage has good secondary dissolved pore development; middle diagenetic stage A2 has acid-base transition belt diagenetic environment, and Huagang formation at the diagenetic stage has weakened dissolution effect and enhanced cementing with decrease of reservoir physical property; middle diagenetic stage B is facing alkalinity diagenetic environment, and the main diagenesis of Huagang formation at the diagenetic stage is compaction and cementing, and affected by this, its reservoir has the worst level. The research considers that lower limit of beneficial diagenetic facies band in vertical direction is basically equivalent to bottom margin of middle diagenetic stage A2 (Figure 4).

Evolution Stage Time Determination
Evolution stage experienced by target layer during geological historical period is divided according to development of overlying formation of target layer. Assumed that target layer L has i layers of overlying formation, which means that overlying formation is marked as follows from the top to the bottom: L1, L2, L3……Li-1 and Li, then target layer has i evolution stages in total during geological historical period, and according to time evolution, and sequential order of evolution stage time of target layer L successively is Li, Li-1, Li-2……L2, L1. From the bottom to the top, overlying formation of Huagang formation of research area respectively is Longjing formation, Yuquan formation, Liulang formation, Santan formation and shallow layer, so Huagang formation has 4 evolution stages in total during geological historical period.

Determination to Burial Depth of Different Stage
Time Based on last step, burial depth of target layer L during different evolution periods is calculated and its computational formula is as follows:

Determination to Formation Temperature of Different
Stage Time In burial process of geologic body, temperature level is expressed as a linear function relationship with depth, and through following temperature calculation model, formation temperature of target layer L at different periods, different depth and different position is calculated; temperature calculation model formula [17,18]: Where, T0 is temperature of normal temperature belt, c is constant, Dep (Ti) is stage burial depth of target layer Ti, H0 is burial depth of constant temperature belt, being constant and T is paleogeotemperature of target layer ( Figure 6).  Finally, in diagenesis simulation process, restricted by current diagenetic stage distribution and type, diagenetic stage distribution and type of reservoir during geological historical evolution period is back stripped through diagenetic temperature to obtain plane distribution diagram of diagenetic stages of the whole research area during each geological historical period (Figure 7).

Determination to Diagenetic Stages of Formation of Different Stage Time
After deposition of Longjing formation, Huagang formation comes to early diagenetic stage A and B (Figure 7 A); after deposition of Yuquan formation, Huagang formation comes to early diagenetic stage A -middle diagenetic stage A1 (Figure 7

Conclusions
(1) Diagenetic change of Huagang formation can be divided into three stages and four stage times, i.e. syndiagenetic stage, early diagenetic stage A and B, and middle diagenetic stage A and B. Diagenetic sequence and diagenetic environment of reservoir sandstone in overall evolution process successively experience: alkalescence of early diagenetic stage A →weak acidity of early diagenetic stage B →acidity-acid-base transition belt of middle diagenetic stage A→alkalinity of middle diagenetic stage B.
(2) When Huagang formation occurs at middle diagenetic stage A1, it has good secondary dissolved pore development; Huagang formation at middle diagenetic stage A2 has weakened dissolution effect and enhanced cementing with decrease of reservoir physical property; main diagenesis of Huagang formation at middle diagenetic stage B is compaction and cementing, and affected by this, its reservoir has the worst level. The research considers that lower limit of beneficial diagenetic facies band in vertical direction is basically equivalent to bottom margin of middle diagenetic stage A2.
(3) After deposition of Longjing formation, Huagang formation enters into early diagenetic stage A and B; at the end of Yuquan formation, Huagang formation is at early diagenetic stage A -middle diagenetic stage A1; at the end of Liulang formation, Huagang formation is at middle diagenetic stage B-early diagenetic stage A; at the end of Santan formation, Huagang formation is still at middle diagenetic stage B-early diagenetic stage A.