Introduction

Resistivity low-contrast oil pay, as an important oil resources with strong concealment, has received much interest in recent years due to their high oil production and wide distribution around the world (Zemanek 1989; Worthington 2000; Onyinye 2010). The resistivity low-contrast oil pay is defined as an oil zone which has small resistivity difference to the water layer, and it can be summarized into two aspects: (1) The absolute value of resistivity in oil layers is low, which can be lower than that of adjacent water layers. (2) The resistivity increase rate of the oil layer is generally between 1.0 and 3.0 (** methods and adaptive analysis of tight sandstone reservoir of Yingcheng formation in Lishu Fault. Adv Earth Sci 31(10):1056–1066" href="/article/10.1007/s13202-021-01195-1#ref-CR15" id="ref-link-section-d199020249e553">2016). On the basis of reservoir characteristics analysis, studying the main influence factors of resistivity low-contrast oil pay from multiple aspects is the key to carry out effective log interpretation and evaluation (Mao et al. 2019) proposed a comprehensive fluid identification method for fluid ty** in the Huanxian area of the Ordos Basin in China by analyzing the regional distribution characteristics of resistivity low-contrast oil pay reservoir.

The tight sandstone reservoir in Longdong West area, Ordos Basin, China, is exploring and develo**. The target bed of this study is Chang 8 tight sandstone reservoir, which is located below the Chang 7 oil shale, and it is the main production horizon of oil and gas in this area due to its good hydrocarbon storage condition (Fig. 1). However, with the deepening of oil exploration and development, more and more resistivity low-contrast oil pays were found, which brings great difficulties to the log interpretation and evaluation of reservoir. Therefore, we present a research to study the reservoir characteristics and control mechanism of resistivity low-contrast oil pay, which aim to provide a valuable reference for the future exploration and development of resistivity low-contrast oil pays in this area.

Fig. 1
figure 1

Location of the research area and target horizon

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Data and research method

Considering that the resistivity low-contrast oil pay and normal oil pay are developed together in this region, so we first separated the reservoir into resistivity low-contrast oil pay (RLP) and normal oil pay (NP) based on the relative value of the apparent resistivity increase rate (ARI), which is calculated by dividing the log data of formation resistivity (RT) by the pure water layer (R0) of the same or adjacent well sections. According to the previous understanding of the definition of RLP reservoir (** of acoustic and resistivity log curves to reflect source rock and the filling color is black-gray, third track is geological stratification, fourth track is logging interpretation conclusion and the fifth track is natural gamma and permeability. It can be seen that the effective thickness of source rock in low resistivity reservoir area is small, and the distance between source rock and reservoir is long. Figure 13b shows the plane distribution of formation resistivity in the Huanxian area, which is located in the northwest of our study area, and the red line represents the location of cross-wells. In order to characterize the hydrocarbon expulsion capacity of source rock, we measured the distance between source rock and reservoir (D), and the effective thickness of source rock (H) based on the cross-well profiles. Then, the hydrocarbon expulsion capacity factor H/D was calculated by dividing H by D to reflect the hydrocarbon expulsion capacity of source rocks. The larger the H/D value, the stronger the oil filling capacity of source rocks to reservoir. Figure 13c shows the relationship between calculated H/D and reservoir resistivity. It can be seen that with the increase of H/D, the reservoir resistivity increases gradually, which indicates that the development of RLP reservoirs also controlled the difference of hydrocarbon expulsion ability of source rock.

Fig. 13
figure 13

source rock on reservoir resistivity: a The cross-well profiles from RLP reservoir to NP reservoir, b the location of cross-well profiles and c the relationship between formation resistivity and the hydrocarbon expulsion ability factor of source rock

The influence of hydrocarbon expulsion capacity of

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Macro-factors: regional difference of formation water salinity

The plane distribution characteristics of formation resistivity and formation water salinity were compared to study the influence of regional difference of formation water salinity on reservoir resistivity. The average values of resistivity log in the test interval were used to represent the reservoir resistivity, and a plane distribution map of formation resistivity is shown in Fig. 14a. The plane distribution map of formation water salinity was plotted based on the formation water salinity analysis data (Fig. 14b). By comparing Fig. 14a and Fig. 14b, it can be seen that in the west of Huanxian area, the formation water salinity is high and the formation resistivity is low, while in the east of Huanxian area, the formation water salinity is low and the formation resistivity is high. This corresponding relationship between formation resistivity and formation water salinity shows that the distribution of RLP and NP reservoirs is also controlled by the regional difference of formation water salinity.

Fig. 14
figure 14

Plane distribution maps of a formation resistivity and b formation water salinity, in which the red solid dots are the location of the wells

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Discussion

According to the analysis of reservoir characteristics, the reservoir main difference between RLP and NP lies in the main types of clay minerals and pore structure. The thin-film chlorite and filamentous illite are the most abundant clay minerals in RLP reservoirs and NP reservoirs, respectively. Although the pore structure of RLP is poorer and the micropores are more developed in RLP reservoir than NP reservoir, the average porosity and permeability are better. Previous researches have shown that the clay mineral is an important factor to affect reservoir physical properties (Durand et al. 2001; samakinde et al. 2016; Yousef et al. 2018). In general, the clay minerals filling in the pores will reduce the porosity and permeability and the pore structure becomes worse. However, if the rock matrix grains are coated by chlorite films, the intergranular porosity will be well protected due to its preventing function for quartz cementation (Chen et al. 2011; Stephan and Stuart, 2016; Higgs et al. 2017). In Fig. 15a, b, we plot the relationship between chlorite and illite and core porosity and permeability. It can be seen that the reservoir porosity and permeability are positively correlated with chlorite content, but negatively correlated with illite content. Therefore, it is considered that good reservoir porosity and permeability of RLP reservoir in our study area are related to the filling of a large number of thin-film chlorite in the pores.

Fig. 15
figure 15

a Cross-plot of core porosity and the content of illite and chlorite and b cross-plot of core permeability and the content of illite and chlorite

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In addition, good porosity but high content of micropores in RLP reservoir indicates that when the filling pressure of crude oil is sufficient enough, the oil-bearing reservoir with high oil saturation can be formed. However, the hydrocarbon expulsion ability of source rock in RLP reservoir is weak and pore spaces are mainly occupied by irreducible water, which makes the oil layer indicating a low oil saturation and low resistivity logging response. In addition, the difference of formation water salinity not only indicates the distribution characteristics of the RLP and NP from the macro-aspect, but also reduces the contrast of resistivity on the micro-aspect. The formation and distribution of RLP in our study area are controlled by the micro-factors and macro-factors together.

Conclusions and suggestions

  1. (1)

    There are differences in main clay mineral types and pore structure between RLP reservoirs and NP reservoirs. Chlorite and illite are the most abundant clay minerals in RLP reservoirs and NP reservoirs, respectively. Although the porosity of RLP is better than NP, the pore space is mainly composed of micropores.

  2. (2)

    The high content of micropores in RLP reservoir makes the irreducible water saturation high, together with the high formation water salinity reduced the reservoir resistivity from reservoir micro-aspect.

  3. (3)

    The difference of hydrocarbon expulsion capacity of source rock as well as the regional difference of formation water salinity is the macro-factors influencing the distribution of RLP and NP.

  4. (4)

    It is suggested to comprehensively consider the micro-factors and macro-factors during the exploration and development of oil resources in our study area.