Logging Characteristics and Identification Methods of Low Resistivity Oil Layer: Upper Cretaceous of the Third Member of Qingshankou Formation, Daqingzijing Area, Songliao Basin, China

This study focuses on low resistivity thick layer sandstone in the X∼XII groups of the third member of Qingshankou Formation at Daqingzijing oilfield, along with comprehensive data of logging, core, oil test, and production test. Based on the current data, we characterized the logs of low resistivity thick-layer sandstone, quantitatively identified calcareous sandstone and low resistivity reservoir, predicted the reservoir thickness, and further explored the causes of low resistivity reservoir of the region.The resistivity of thick layer sandstone in the X∼XII groups of Qingshankou Formation can be classified into low amplitude logfacies, middle amplitude logfacies, and sharp high amplitude logfacies. Sharp high amplitude logfacies sandstone is the tight sandstone of the calcareous cementation. Low amplitude logfacies sandstone is water layer. For themiddle amplitude logfacies sandstone, water layer or oil-water layer can be identified with the identification standard. Low amplitude structure, high clay content, high irreducible water saturation, and high formation water salinity are attributed to the origin of low resistivity oil layer.


Introduction
Songliao Basin is a large Mesozoic to Cenozoic hydrocarbonbearing sedimentary basin in Northeast China (Figure 1(a)), with a total area of about 260,000 km 2 . The basin is diamond shaped with its long axis oriented NNE. Six secondary structural units are present in the basin, including the central concave, the southwest uplift, the southeast uplift, the northeast uplift, the western slope, and the northern dip areas [1,2]. Previous studies showed that this basin is filled with continentally associated clastic strata, with multiple oilbearing layers, of which Cretaceous and Tertiary lacustrine units are the most important [3][4][5][6].
The Daqingzijing oilfield is located in the south central concave of Songliao Basin (Figure 1(b)). The overall structure is an asymmetric syncline with axial NNE direction that has a steeply dipping west limb and nearly horizontal east limb. It has a practical exploration area of 1500 km 2 that possesses reserves of approximately 700 million barrels of crude oil. The main reservoir unit is the Cretaceous Qingshankou Formation wherein the depth of oil formation ranges from 1600 m to 2500 m. The Qingshankou Formation is characterized by low porosity, low permeability, and low resistivity sandstone reservoirs [7]. Reservoir microfacies, resource distribution, and identification of low resistivity resource accumulations are urgent areas of research for this oilfield [7][8][9][10].
The identification and evaluation of low resistivity reservoirs are technically challenging [11][12][13][14][15][16]. This study focuses on the low resistivity thick sandstone in the X∼XII groups of the K 2 qn 3 (the third member of Qingshankou Formation) at Daqingzijing oilfield and summarizes petrophysical welllog response characteristics and its logfacies of low resistivity  thick layer sandstone based on comprehensive well-log, core, oil, and production test data. The petrophysical interpretations of the different types of logging curve are discussed. A secondary aim of this paper is to establish quantitative parameters for identification of the calcareous sandstone, low resistivity reservoir, and reservoir thickness and further understand the causes of low resistivity reservoir in the area. The results will provide guidance for the identification of the calcareous sandstone and low resistivity oil reservoirs elsewhere.

Geological
Yi'an Formation (E 2 y), Da'an Formation (Nd), and Taikang Formation (Qt) [17][18][19]. The Daqingzijing oilfield has four oil-bearing horizons: the Fuyu, Gaotaizi, Putaohua, and Heidimiao Formations. The main oil-bearing level is Gaotaizi reservoir of the Qingshankou Formation (K 2 qn) (Figure 2(a)) [20]. There are three members that comprise the Qingshankou Formation: these are referred to as K 2 qn 1 , K 2 qn 2 , and K 2 qn 3 . Regarded as a deeply buried lacustrine unit, the lithology of K 2 qn 1 is dominated by graydark shale, gray mudstone, argillaceous siltstone, and siltstone; it includes four sand-dominated levels, which range in thickness between 70 and 100 meters. The lithology of K 2 qn 2 is mainly dominated by gray mudstone, argillaceous siltstone, and siltstone representing delta front sedimentary environment; it includes five sand-dominated levels, which range in thickness between 160 and 200 meters. The lithology of K 2 qn 2 is mainly dominated by gray mudstone, argillaceous siltstone, siltstone, and sandstone representing distal lacustrine and delta front sedimentary environments (Figure 2(b)) [21].

Lithology and Physical
Property. Based on the identification and analysis of thin sections, lithologies of the X∼XII sandstones of K 2 qn 3 Formation are identified as lithic arkose and feldspathic lithic sandstone, of which the average content of quartz is 38.5%, feldspar is 51.6%, and lithic fragments are 36.9% (Figure 3(a)). Core logging shows that the X∼XII sand groups of K 2 qn 3 Formation consist of calcareous siltstone, calcisiltite, argillaceous siltstone, and siltstone ( Figure 3(b)).
Statistical analysis of 336-core sample from 8 cores in the area shows that the porosity of the sandstone of the X∼XII sand groups of K 2 qn 3 Formation ranges within 8%∼17% with an arithmetic mean of 11.7% (Figure 3(c)). The permeability is distributed within 0.02∼160 × 10 −3 m 2 with average value of 4.5 × 10 −3 m 2 (Figure 3(d)), all of which indicates that K 2 qn 3 is a typical low porosity and low permeability reservoir.

Well Logging Curve Types of Thick Layer Sand Bodies.
Distributary channel sand bodies have been interpreted to reside in the delta front sedimentary environment of the study area [7] (Figure 2). Channels are locally amalgamated into sandstone bodies that are >15 m thick. The GR curves of the thick sandstones are box type with abrupt changes at top and bottom ( Figure 4). In this area a reasonable cut-off for sandstone versus mudstone is 90API. The RILD (deep induction logging resistivity) and RILM (medium induction logging resistivity) curves of the thick sand bodies are mainly manifested as one of the following 3 profiles. (1) Low amplitude logfacies proffer a low amplitude, nearly flat curve, indicating that the resistivity of the sandstone is similar to the resistivity of overlaying mudstone section (Figure 4(a)), with the average value of RILD between 4.4 and 10 Ω⋅m (Table 1). (2) Middle amplitude logfacies near the middle of the logarithmic induction scale again produce a flat curve, indicating that the resistivity of the thick sand bodies is higher than the resistivity of overlaying mudstone section and the low amplitude sandstone (Figures 4(b) and 4(c)), with the average value of RILD between 7.5 and 13.7 Ω⋅m (Table 1). (3) Sharp-high amplitude logfacies refer to the resistivity of the thick sand bodies that is apparently sharp higher than the resistivity of low and middle amplitude kind (Figures 4(a) and 4(b)), with the average value of RILD between 7.4 and 23.9 Ω⋅m (Table 1). The sharp-high amplitude induction response is mainly positioned at the top or bottom of thick sandstones (Figures 4(b) and 4(c)). The above 3 kinds of curves of the thick layer sand bodies can be merged together, which are presented as follows: (1) sharp-high, low, and sharp-high amplitude combination (Figure 4

Electrical Characteristics of Thick Layer Sand Bodies.
Resistivity of thick layer sand bodies of eleven oil test and production wells is shown in Figure 5. The results showed that the RILD and RILM curves present the characteristics of double peak feature. The first peak of RILD is 12 Ω⋅m; the second peak of RILD is 28 Ω⋅m (Figure 5(a)). The first peak of RILM is 10 Ω⋅m; the second peak of RILM is 28 Ω⋅m (Figure 5(b)). Overall, the resistivity of the thick sand bodies is low; as such we characterize it as low resistivity reservoir. Identification of quantitative relationship between the double peaks feature of the three types of logfacies and the variable well-log response makes interpretation of the reservoir fluid content problematic.

Identification of Calcareous
Interlayers. The RILD and RILM curves are characterized by sharp-high amplitude at the top and bottom of the thick layer sand bodies ( Figure 4). There are two likely explanations of the increase of reservoir resistivity: (1) there is oil or natural gas in the sandstone or other nonconducting fluid or gas [22,23]; (2) the reservoir is relatively tight (tight reservoir), therefore leading to the increase of resistivity and decrease of acoustic time simultaneously [24]. The AC value of the sharp-high amplitude of The second member (K The first member The third member (K thick layer sand bodies decreases compared to the middle and low amplitude section ( Figure 4 and Table 1). Therefore, the section of sharp-high amplitude (resistivity increases and the acoustic time decreases) likely indicates a tight sandstone section.
Based on the identification of the core samples through the scanning electron microscope, calcite cements are common in the study area ( Figure 6). The carbonate content of thick sand bodies in five cored wells ranges from 0.8% to 58.3%, with the average value of 13.1%; the carbonate cement content of most samples is distributed within 0.8%∼20% (Figure 7(a)). Normally, the impact of calcite cements on the porosity of sandstone is relatively small when calcite < 25%. However, above 25%, carbonate crystals fill the void space and pore throats with poikilitic cement. The porosity of sandstone reservoir decreases with increasing carbonate content (Figure 7(b)), leading to increased resistivity and decreased acoustic time. In short, carbonate cementation is the main factor that leads to sharp-high amplitude at the top and bottom of the thick sand bodies in the study area.
As the increase of the resistivity value is mainly caused by carbonate cementation rather than oil or gas in sandstone reservoirs, the sharp-high amplitude of calcareous sandstone section should be identified separately during the process of identifying the oil layer and the water layer based on the resistivity value. Based on the statistics of RILD of sharphigh, middle, and low amplitude of thick layer sand bodies from ten oil tests and production wells, the average RILD of sharp-high amplitude ranges within 7.4 Ω⋅m∼26.4 Ω⋅m with maximum distributed within 9.1 Ω⋅m∼30.3 Ω⋅m. The average RILD of middle amplitude ranges within 7.5 Ω⋅m∼14.1 Ω⋅m, while the average RILD of low amplitude ranges within 4.4 Ω⋅m∼10 Ω⋅m (Table 1). As shown in the cross plot of RILD and AC of the three above types, there is a significant RILD overlap within 7 Ω⋅m∼15 Ω⋅m for the sharp-high and middle amplitude, as well as AC overlap within 230 s/m∼245 s/m (Figure 8(a)); therefore, it is not practical to identify the calcareous interlayers by using RILD and AC cross plot.
Because the sharp-high, middle, and low amplitude curve types are all based on the comparison of the RILD to that of the adjacent sandstone section, two new parameters, JRT and JAC, were constructed to characterize the relative variability. The specific formula is as follows: JRT is relative resistivity, dimensionless; RILD max is maximum RILD of sandstone section in the adjacent sharp-high amplitude, Ω⋅m; RILD ave is average RILD of sandstone section in the sharp-high, middle, and low amplitude, Ω⋅m; JAC is relative acoustic, dimensionless; AC min is minimum AC of sandstone section in the adjacent sharp-high amplitude, s/m; AC ave is average AC of sandstone section in the sharphigh, middle, and low amplitude, s/m; JRT and JAC were calculated according to (1) for sharp-high, middle, and low   amplitude of sandstone section ( Table 1). As indicated from the JRT and JAC cross plot, JRT value of the sharp-high amplitude sandstone is less than 1.4, and the JAC value is greater than 0.88, while the JRT value of middle and low amplitude sandstone is greater than 1.4, and the JAC value is less than 0.88 (Figure 8(b)). The cross plot of JRT and JAC can clearly distinguish the sharp-high, middle, and low amplitude sandstone and identify the sharp-high amplitude calcareous sandstone.

Identification of Low Resistivity Oil Layers.
Based on the statistics of the RILD of middle and low amplitude of thick sandstone in ten oil test and production well, the average RILD in the middle amplitude sandstone section is between 7.5 Ω⋅m and 14.1 Ω⋅m, while in the low amplitude sandstone section it is between 4.4 Ω⋅m and 10 Ω⋅m (Table 1), leading to a significant overlap for the RILD of middle and low amplitude sandstone in the range of 7.5 Ω⋅m∼10 Ω⋅m; this makes it hard to identify the oil and water layer with only  RILD. For example, in the H24 well, the average RILD in the middle amplitude sandstone section is 8.2 Ω⋅m, and the test data proves it as water layer, while in the Qx110-36 well, the average RILD in the middle amplitude sandstone section is 13.7 Ω⋅m, but the test data proves it as oil-water layer ( Table 2). The data from eleven test and production wells in the study area show that the average RILM is greater than the average RILD in water layer sandstone (Table 2), such as in H15 (Figure 4(a)) and H24 (Figure 4(c)). The average RILD is greater than the average RILM in oil layer sandstone (Table 2), such as in Qx94-26 (Figure 4(b)) and Qx110-36 (Figure 4(d)). This is due to a fact that the formation of water resistivity is less than that of mud filtrate, and the mud filtrate resistivity is less than that of oil layer. When the mud filtrate invades into the water layer, the resistivity of the mixed formation water in the intrusive zone is certainly greater than that of the water layer, which explains why RILM value is greater than RILD. Moreover, if the mud filtrate invades into oil layer, the resistivity of the mixed liquid in the invasion zone is expected to be less than that of the oil layer, and consequently RILD is greater than RILM.
Based on the statistical results of resistivity of the test and the production data (Table 2), the RILD and RILM cross plot is regarded as a standard resistivity identification method to distinguish oil-water layer from water layer, whereas the average value of RILM is difficult to distinguish the oil-water layer from water layer ( Figure 10). The average RILD is greater than 10.4 Ω⋅m in oil-water layer and less than 10.4 Ω⋅m in water layer ( Figure 10, Table 2), because the average RILD of  Oil-water layer low amplitude sandstone is less than 10 Ω⋅m (Table 2), so low amplitude type sandstones are all water layer; if the average RILD of middle amplitude sandstone is greater than 10.4 Ω⋅m, it is oil-water layer, or else, it is water layer ( Table 1). Based on the deduction of the sharp-high amplitude calcareous interlayers, the fluid identification standard of thick layer sand bodies is as follows: (1) if the sandstone is low amplitude type and ΔRT (RILD-RILM) < 0, it is water layer; (2) if the sandstone is middle amplitude type, the average RILD < 10.4 Ω⋅m, and ΔRT (RILD-RILM) < 0, it is water layer; (3) if the sandstone is middle amplitude type, the average RILD > 10.4 Ω⋅m, and ΔRT (RILD-RILM) > 0, it is oilwater layer.

Discussion
Sandstone in the X∼XII sand groups of K 2 qn 3 at Daqingzijing oilfield is regarded as a typical low resistivity oil layer. Genetic factors of low resistivity oil layer are very diverse, with low amplitude structure [25], sedimentary facies [26], interstitial materials [27], irreducible water [28], formation water salinity [29], drilling mud invasion, and other factors. This paper will discuss four aspects: low amplitude structure, interstitial material, irreducible water, and formation water salinity.   curve type has various values ( Table 2); because the reservoir of the study area is low structure reservoir, the oil column height is less than 20 m. Different area of drilling wells in the oil reservoir is considered as oil-water layer if the sand body was on top of the oil-water interface and characterized by middle amplitude curve type; the thickness of the middle amplitude curve segment is the oil layer thickness (Figures  1(c) and 11, Qx110-36). However, it is considered as water layer if the sand body was under the oil-water interface and characterized by low amplitude curve type (Figures 1(c) and 11, Qx110-38, Qx110-40). Moreover, the sharp-high type sandstone because of calcareous interlayer is distributed at the top of the thick layer sand body and is effective cap rock.

Characteristics of Interstitial Materials.
Through a quantitative analysis of X diffraction data, the total amount of clay minerals of the low resistivity oil layer of K 2 qn 3 varies within 1.8%∼5.8%, with an average of amounts to 4%. The clay mineral compositions are mainly dominated by illitemontmorillonite mixed-layer and illite, with high cation exchange adsorption capacity and strong conductivity. In addition, most of the cementing materials were subject to argillization due to the epigenesis, which may provide a desirable condition for the existence of a large number of irreducible water [30], therefore forming a good conductive system that reduces reservoir resistivity.

Characteristics of Formation Irreducible
Water. Based on the NMR data in well Q157-19-9, T2 cut-off method showed that the irreducible water saturation is around 30%∼65%. High irreducible water saturation makes the formation serve as an effective molecular conductive network. This leads to a relatively low resistivity of the oil formation in a fresh water environment ( Figure 12).

Salinity Characteristics of Formation
Water. The formation water salinity in the Daqingzijing area of K 2 qn 3 differs largely, ranging from 23662.4 mg/L to 28317.1 mg/L, with a mean value of 26257.6 mg/L. Statistical results of formation water salinity showed that the regional salinity is relatively high; this indicates that the low resistivity reservoir is related to the high formation water salinity of K 2 qn 3 Formation (Table 3).

Conclusion
(1) The resistivity of the thick layer sand body of the study area is mainly distributed within 6 Ω⋅m∼32 Ω⋅m and is featured as double peak; this indicates that the thick layer is a typical low resistivity reservoir. The resistivity curves of the thick layer sand bodies include three logfacies: low, middle, and sharp-high amplitude, and each of them can be merged together, which are presented as the following 4 types: sharphigh, low, sharp-high amplitude combination; middle, sharphigh combination; sharp-high, middle, low, sharp-high amplitude combination; and sharp-high, middle, low amplitude combination.
(3) The features of low amplitude structure, high clay mineral content, high irreducible water saturation, and high formation water salinity in this study area all make the formation as an effective molecular conductive network. This greatly enhances the conductibility and leads to the low resistivity of the reservoir.

Conflicts of Interest
The authors declare that they have no conflicts of interest.