Application of Carbon-Isotope-Logging Technology in High-Temperature and High-Pressure Wells: A Case Study of the Ledong Gas Field in the Yinggehai Basin

Abstract

Carbon isotope logging technology can obtain timely and accurate hydrocarbon fluid and reservoir geological information and has great application potential in oil–gas body property analysis, the comprehensive study of source rocks, and fault-sealing evaluation. Since 2014, real-time methane isotope logging technology has been applied in the western South China Sea. Based on the on-site, real-time, continuous, and accurate detection of methane carbon isotopes, combined with the rapid comprehensive analysis and evaluation of logging gas measurement data, the application effect in ultra-high-temperature and high-pressure wells in the western South China Sea has been remarkable. Taking the Ledong 10 Area of Yinggehai Basin as an example, real-time carbon isotope logging data can be used to quickly identify gas origins, source rock maturity, and gas source type and help judge the sealing quality of overburdened mudstone caps. This knowledge can serve as a reference for ascertaining the popularity of isotope-logging technology in other areas.

1. Introduction

Methane is a hydrocarbon natural gas composed of carbon and four hydrogens, and its composition is relatively simple, so the carbon isotope composition of methane can be used as a tracer of gaseous hydrocarbons [1]. Carbon isotope data on methane have been widely used in oil and gas exploration and development. The study and analysis of methane carbon isotopes in natural gas deposits can provide information on the origin, type, maturity, migration, and secondary changes of natural gas, aspects that play an important guiding role in oil and gas exploration [2,3]. However, in the past, offshore isotope analysis, a method consisting of sampling on site and returning the samples to a laboratory on land for analysis, was usually adopted. This process was time-consuming and delayed the application of isotope data in oil and gas exploration [4]. In order to meet the needs of oil exploration technology, French Geological Services, a subsidiary of Schlumberger, has developed a logging technology for the on-site, real-time, and continuous detection of methane carbon isotopes in drilling fluids. Through this technology, methane carbon isotope content can be rapidly determined in the field, and information on the origin, type, and maturity of source rocks of natural gas can be provided, providing an important basis for gas, oil source correlation, multi-well correlation, and fault-sealing research [5]. Compared with the previous analysis methods, this field logging technology can realize real-time continuous detection, greatly shorten analysis time, improve the accuracy and reliability of data, and allow methane carbon isotope data to be applied to oil and gas exploration in a more timely manner [6].

With the continuous expansion of oil and gas exploration in the western waters of the South China Sea into the middle and deep areas, complex reservoir conditions, such as low porosity and low permeability, low resistivity, high temperature and high pressure, ultra-high temperature and high pressure, and deep water, are increasingly emerging [7]. Therefore, on the basis of a comprehensive analysis of previous research results, we systematically dissected different types of traps and reservoirs in the diapiric layer and summarized the mechanism of oil and gas migration and accumulation in order to provide a scientific basis for further exploration and development in this area. Previous studies have shown that the natural gas in Yinggehai Basin mainly migrates vertically through the bottom diapir [8]. However, due to the insufficient development of the skeleton sand body, the lateral migration path to the middle and deep strata of the slope zone is not smooth, and the slope zone lacks obvious vertical migration channels such as through-source faults, thus limiting the supply of hydrocarbon sources. In recent years, because of the accumulation of drilling data and the improvement of seismic interpretation technology, the mechanism of oil and gas accumulation in the diapiric fault zone has been recognized. In the middle and deep lithologic gas reservoir with a non-diapir structure in Yinggehai Basin, the discovery of the Ledong 10 gas field has aroused great attention to the problem of gas migration and accumulation (Figure 1) [9]. Especially in the ultra-high-temperature and high-pressure Wells in the Ledong 10 area, the temperature is as high as 150–249 °C, the pressure gradient is as high as 1.90~2.30 MPa/m, and the maximum mud gravity is as high as 2.38 MPa/m. Due to the narrow pressure window, kick and loss often occur. It is difficult to evaluate this reservoir due to the high risk of drilling and distortion of gas measurement data. In this paper, combined with the application of methane carbon isotope data in the Ledong 10 area, on the basis of quickly obtaining the origin, type, and maturity of the source rock of the reservoir natural gas, we will conduct further analysis and gain an understanding of hydrocarbon formation in this reservoir.

The Miocene Sanya Formation and Meishan Formation in Yinggehai Basin are the main source rocks in the basin, mainly composed of shallow Marine mudstone [10]. The Ledong Diapiric zone is located in the center of the main hydrocarbon depression and has a strong gas source base. In recent years, in the southern section of the Yinggehai Basin depression slope zone (that is, the transition area between the central diapir structural zone and the Yingdong slope zone), drilling activities have revealed thick gas layers in the LD10-1, LD10-2, and LD10-3 gas fields in the Ledong 10 area and LD16 gas-bearing structure. These gas layers are located in the high-temperature and high-pressure sections of the Huangliu and Meishan formations. At present, the temperature range of high-temperature and high-pressure drilling is 150~249 °C, the pressure equivalent density is 1.90~2.30 g/cm³, and the maximum pressure equivalent density is 2.38 g/cm³. The ultra-high-pressure-formation pressure system in the Ledong 10 area is complex, and there are large differences between the sand bodies in the target layer [11]. An absolute error of 0.05 may lead to drilling failure. As can be seen from the seismic profile, due to the influence near the diapiric structure, operators have always paid attention to the significant control risk and reservoir physical properties caused by the high pressure of other sources during drilling. In this paper, the above problems are discussed based on the real-time logging of methane and carbon isotopes from several oil wells in the Ledong 10 area.

2. Geological Setting

The Yinggehai Basin is a large Cenozoic strike-slip extensional basin in the northern South China Sea with an area of 11.3 × 10⁴ km² and a maximum Cenozoic sedimentary thickness of more than 17 km. The Yinggehai Basin can be divided into three first-order structural units, namely, the Yingdong slope, the central depression, and the Yingxi slope, of which the central depression can be further divided into the Hanoi Depression, the Lingao uplift, and the Yinggehai Depression. A series of en echelon diapiric structures developed in the central depression under the control of late strike-slip pull-apart mechanisms. The Cenozoic section includes the Yacheng and Lingshui formations of Oligocene origin; the Sanya, Meishan, and Huangliu Formations of Miocene origin; and the, Yinggehai and Ledong Formations of Pliocene origin. The Yacheng Formation and Lingshui Formation were only partially revealed by drilling in the Lingao uplift and slope zone [12].

At present, the natural gas discovered through exploration is mainly distributed in the diapir structure development area or the diapir structure spreading area, and some of it is also found on the Yingdong slope and in the southern uplift area of the central depression. The Ledong and Dongfang areas are located on the eastern part of the Ledong slope, that is, the transition zone between the southeastern part of the central depression and the Yingdong slope, and they are a certain distance from the diapiric development area [13]. The formation pressure coefficient and temperature in the study area gradually increase with the buried depth. When the buried depth exceeds 3000 m, the formation pressure coefficient reaches 1.5; when the depth approaches 4000 m, the formation pressure coefficient increases to 2.0, and the maximum formation pressure coefficient reaches 2.3. The present geothermal gradient of the Yinggehai Basin is 1.89~5.66 °C/km, and the main distribution range is 3.00~4.00 °C/100 m. The geothermal gradient of the middle and deep layers in the study area exceeds 4.50 °C/100 m, and the temperature of the inner layer of the reservoir is 158.2~189.6 °C. Natural gas is mainly enriched in the reservoir cap combination of the Meishan Formation and Huangliu Formation at moderate and deep depths, and the burial depth is great. The reservoir’s lithology is mainly silty fine sandstone and medium sandstone, with a porosity ranging from 7.5% to 13.8% and a permeability ranging from 0.622 to 48.600 mD. The large amount of mudstone and silty mudstone in the second member of the overlying Yinggehai Formation and the first member of the Huangliu Formation is the stable regional cap layer in the study area, and the deep, large-scale faults connect the source rock and reservoir, providing favorable conditions for gas enrichment [14].

3. Material and Methods

The working principle of real-time isotope-logging technology is divided into two parts: isotope measurement and component concentration measurement [15].

The real-time methane carbon isotope measurement principle depends on the near-infrared light absorption principle. The absorption of infrared light by atoms (or isotopes) of different masses is selective. ¹²C and ¹³C have different masses, so they absorb different wavelengths of near-infrared light. Regarding the principle of real-time component concentration measurement, the cavity ring-down spectroscopy (CRDS) principle is adopted. A laser pulse of a fixed wavelength will have a ring-down effect in an optical cavity filled with hydrocarbon gas [16].

The Grand-3 methane carbon isotope-logging instrument produced by Suzhou Guande was used in this study. The working methane concentration was greater than 0.1%, the sampling frequency was 4 Hz, and the standard deviation of five measurements was less than 0.25‰. At the worksite, the real-time isotope equipment could be used directly with the field logging gas equipment, without the need for a separate degasser. Existing on-site logging resources can be leveraged to optimize cost efficiency (Figure 2) [17].

Prior to conducting the analysis during drilling, the instrument was calibrated using a certified standard gas sample to ensure that the results obtained were consistent with the laboratory data.

In traditional isotope determination techniques, on-site samples are taken and returned to a land laboratory for analysis. This approach takes a long time and leads to transport problems. Additionally, this scattered form of analysis is not continuous, and sample quality is affected by sampling personnel, gas leakage, and condensation [10]. Real-time isotope logging technology can effectively solve these problems, and the corresponding technical characteristics are as follows: (i) The equipment has a drawer structure and a small volume and is easy to install and carry on site. (ii) It can be used directly with logging gas equipment without a separate degasser. (iii) It enables continuous and real-time sample analysis. (iv) It allows strict quality control (QC) [18].

Prior to conducting the analysis during drilling in well X-1 (Figure 3), the instrument was calibrated using a certified standard gas sample to ensure that the results obtained were consistent with the laboratory data.

4. Result and Discussion

4.1. Gas Origin, Gas Source Type, and Source Rock Maturity of Oil and Gas Reservoir

Studying the origin of natural gas and accurately dividing gas reservoirs constitute the key to gas exploration and production. Carbon isotope logging is a very economical and effective method in the study of natural gas genesis [19]. The Bernard chart of natural gas genesis, proposed by Bernard and Whitkar [20], is widely used to study natural gas genesis using carbon isotopes of methane. The ratio characteristics of δ¹³C₁ and methane/(ethane + propane) are used to divide natural gas genesis into three categories: biological origin, mixed origin, and thermal evolution origin [21].

The variation range δ¹³C of methane in the main gas anomaly section of 3382.00–3412.00 m in well X-2 is between −39.72‰ and −38.70‰, with an average value of −39.26‰. According to the isotope data corresponding to the target interval on the Bernard chart (Figure 4), the origin of natural gas can be considered thermal [22].

At the same time, we projected 3382.00–3412.00 m methane isotope data combined with logging gas component data on a map of Dai Jin (Figure 5), which falls in the oil-type gas–coal-type gas region.

Vitrinite reflectance is represented by the symbol R_o. Vitrinite is a component of coal that consists mainly of aromatic, thick-ring compounds, but the plant tissue in it cannot be directly observed. As the degree of coalification increases, the degree of condensation of aromatic structures in vitrinite also increases, resulting in an increase in reflectance [24]. The process of the evolution of vitrinite is closely related to the thermal-cracking process of the oil-generating parent material, so it is a good indicator of the maturity of organic matter. The reflectance of vitrinite increases with the increase in the thermal metamorphism of organic matter. It is generally believed that a reflectance of vitrinite between 0.5% and 1.35% indicates the mature zone of petroleum, while a reflectance of less than 0.5% indicates the immature zone, and more than 1.35% indicates the over-mature zone [25].

The carbon isotopes of methane are closely related to the maturity (R_o) of the source rocks, and many domestic and foreign scholars have proposed corresponding regression equations for different blocks. In China, the δ¹³C₁—R_o equation is commonly used:

Oil–gas regression equation: δ¹³C₁ = 15.8 × lgR_o − 42.2 (according to Dai Jinxing) [26].

The specific process data are shown in

Table 1. Methane isotope log analysis table of the abnormal section of well X-2.

Well Interval (m)	Variation Interval of δ13C-CH4 (‰, V-PDB)	δ13C-CH4 (‰, V-PDB)	Maturity (Ro)	Gas Type
3382.00–3412.00 m	−39.72~−38.70	−39.26	1.53	Over-mature oil type gas-coal type gas

. The comprehensive analysis results of well interval 3382.00–3412.00 m indicate the presence of over-mature oil-type gas and coal-type gas.

The existing research findings suggest that the primary hydrocarbon-generating strata in the Qiongdongnan Basin are the coal-measure source rocks of the Yacheng Formation, predominantly in the over-mature stage. Notably, the area near the Ledong Gas Field exhibits a maturity ratio of 1.4–2.0, corroborating the results of MWD analysis [27].

4.2. Identification of the Source of High Pressures via Isotope and Gas Logging

An accurate understanding of the cause of abnormal high pressures during drilling can provide theoretical support for the accurate monitoring of abnormal formation pressure and guide the adjustment of drilling fluid density in a rational manner so as to ensure safe drilling operations and protect oil and gas reservoirs. Based on the analysis of methane carbon isotope data obtained during drilling, a correlation between the morphological characteristics of the methane carbon isotope curve and the causes of different abnormal high pressures was established, and the source of abnormal overpressure was judged according to the real-time methane carbon isotope data so as to improve the monitoring accuracy of formation pressure while drilling [28].

Application 1: After the hole being dug for well X-3 entered the second member of the Huangliu Formation at a depth of 3846.00 m, the carbon isotope composition of methane suddenly increased, obviously deviating from the original upward trend and being significantly different from the sedimentary environment of the formation, indicating that there was deep fluid intrusion, resulting in an in increase in the carbon and methane carbon isotope values of the formation fluid. However, the overlying layer of the sandstone layer had no obvious gas invasion characteristics, and the gas measurement value of the mudstone cap on the top of the high-pressure sandstone tended to be stable. Combined with the above analysis, it can be inferred that this may be because the type of pressure of the sand body belongs to the conduction/fluid expansion category, belonging to the other source type of high pressure, which has a short high-pressure compression period and insufficient pressure and material transfer between the high-pressure sandstone and the upper cap. There was an obvious methane carbon isotope surge trend [29].

The methane carbon isotope value of well X-4 shows an increasing trend with depth, without any significant abrupt changes. Its value matches the characteristics of the sedimentary layer where it is located, demonstrating typical autochthonous high-pressure features (Figure 6). However, before approaching the sandstone section, the overlying mudstone caprock shows obvious signs of gas invasion, and the gas-logging value of the mudstone caprock at the top of the sandstone gradually increases. Based on the above analysis, it can be inferred that the pressure formation mechanism of the high-pressure sand body may be due to undercompaction, corresponding to the autochthonous high-pressure type. Due to the long pressure-formation period of autochthonous high pressure, there is sufficient pressure transmission and material exchange between the high-pressure sandstone and the upper caprock, which leads to gas invasion. In addition, there are no significant abnormal changes in the methane carbon isotope throughout this depth range (Figure 7).

Therefore, before the sand body of the high-pressure reservoir is exposed, if the gas logging of the overlying mudstone shows no anomalies but the isotopes show a significant upward trend, a pre-judgment of the foreign high-pressure source can be made by integrating an analysis of the isotopic gas source type so as to guide the formulation of an operation plan before the sand body is opened, ensuring the safety, quality, and efficiency of operations in an ultra-high pressure formation [30].

Application 2: Before drilling into the reservoir sand body of well X-3 at 3382.00–3412.00 m, the logging gas measurement of the overlying mudstone showed no abnormalities, but the isotope showed a significant upward trend. This sudden change in the isotope with depth was attributed to the intrusion of external fluids. Considering the geological conditions around the wellbore, it is believed that the type of isotope gas source, oil-type gas–coal-type gas, is influenced by the structure of the Lüdong 15-1 diapir. Based on regional characteristics and the adjacent X-4 well, it was determined that deep fluids (high pressure from other sources) have entered the sand body [31]. We can suggest that operators prepare a well control plan in advance, as there is a risk of high pressure from other sources in the lower sand body. When drilling to 3382.00 m, the highest total gas measurement was 25.9%, the levels of the background gas of the mud remained high, the mud density increased from 1.88 to 1.95, and the pressure returned to normal. The methane isotope value was −39.26‰, and the gas source type was over-mature oil-type–coal-type gas [32].

4.3. A Sharp Increase in Isotope Values Indicates Good Sealing Properties of the Cap Rock

Understanding: The trend in which the carbon isotope values surge reflects the sealing quality of the overlying mudstone reservoir. The larger the surge trend, the better the mudstone sealing, and the higher the oil and gas saturation of the reservoir. High formation temperatures, high formation pressures, and a narrow pressure window often cause kick and loss in high-temperature and high-pressure wells in the Ledong 10 area. It is difficult to evaluate reservoirs with distorted gas measurement data. It is a difficult problem to interpret and evaluate the reservoir. The composition of carbon isotopes under a methane surge trend can be used as a new reference to evaluate the gas abundance of reservoir sand bodies [33].

The carbon isotope composition of methane at the base of the upper mudstone within the Meishan Formation 1 reservoir sandbody in the target layer of well L9 increased by nearly 90‰, whereas in well L6, the methane isotope value at the base of the upper mudstone section of the Meishan Formation 1 reservoir sandbody showed a gradual increase starting from the second member of the Huangliu Formation (Figure 8) [34].

It can be seen from the comprehensive logging interpretation and correlation diagram (Figure 9) in well X-5 that the carbon isotope composition of methane at the bottom of the mudstone section in the upper part of the reservoir sand body increased sharply. Additionally, the state of the reservoir formation is good, and the first member of the Meishan Formation showed a large gas suite. On the contrary, in well X-6, the methane carbon isotope values at the bottom of the mudstone section in the upper part of the reservoir sand body increased slowly, the reservoir accumulation was poor, and the gas saturation of the first member of Meishan Formation was low and dry [35].

4.4. Isotopic Trends Support Drilling Operations

As well X-1 is adjacent to the Ledong 15-1 Diapir and highly affected by diapir fluid, real-time methane carbon isotope logging technology was used to obtain formation fluid information to help evaluate flow connectivity [36]. In Figure 5, it can be observed that the δ¹³C values of the upper Ledong Formation and Yinggehai Formation underwent abnormal changes, ranging from −65‰ to −35‰. According to the obvious negative slope characteristic of the gas measurement curve, it was inferred that the well may be affected by water. The carbon isotope values reveal that the types of gas sources are oilfield gas and condensate gas, which are not consistent with the biogenic gas source type of the formation. Combined with regional characteristics, it was inferred that there is a permeable formation of deep fluid entering this well section in this formation, but it was not possible to determine whether the fluid migrated upward from the Ledong 15-1 diapir zone to the Yinggehai Formation laterally into this block or whether there is a vertical channel directly in this block. Due to the great thickness of the overlying strata and the complex and diverse lithology of the block, it was necessary to fracture the layer. In the presence of small fracture zones, the risk of leakage prevention in the lower formation is significantly increased. To this end, the entire drilling process was recorded in detail in the field application, a comparative analysis with the adjacent well was carried out, and timely reports were made to the operator to formulate appropriate countermeasures [37]. During the drilling process, the gas measurement data were analyzed, and the correctness of the inferred results was verified via tests.

This technology further enriches the technical means of logging, provides a more reliable way to explain and evaluate the nature and genesis of gas reservoirs, allows in-depth studies of the origins of gas, enables the analysis of the maturity of source rock, and facilitates the comparison of oil and gas sources, thereby providing a more comprehensive understanding. With the continuous improvement of modern analytical technology, the research and development and application of carbon isotope logging technology for heavy hydrocarbons (such as ethane, propane, butane, etc.) will continue to advance. Carbon isotope logging will play an important role in hydrocarbon genesis, oil and gas correlation, mixed oil and gas identification, and oil and gas tracking, and the application of this technology will make contributions to oil and gas exploration and development.

5. Conclusions

Real-time methane carbon isotope-logging technology has been widely used in the western oil region of the South China Sea. Through the analysis of real-time methane isotope data in the Ledong 10 area and gas measurement data, the following conclusions were obtained:

• Real-time carbon isotope logging can quickly identify gas origin, maturity, and gas source type.
• Operational plans should be made before opening a sand body guided by real-time carbon isotopes of methane; the safety, quality, and efficiency of ultra-high-pressure formation operations should be ensured; and efforts should be made to help reduce costs and increase efficiency.
• According to the methane carbon isotope logging of the mudstone cover overlying a reservoir sand body, the sealing quality of the overlying mudstone cover can be employed to help make judgements so as to evaluate the gas abundance of the reservoir sand body as a new reference [8].