Article pubs.acs.org/EF
Evaluation of Lower Cambrian Shale in Northern Guizhou Province, South China: Implications for Shale Gas Potential Shuangbiao Han,*,†,‡ Jinchuan Zhang,† Yuxi Li,§ Brian Horsfield,‡ Xuan Tang,† Wenli Jiang,§ and Qian Chen† †
School of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China German Research Centre for Geosciences (GFZ), Potsdam 14473, Germany § Strategic Research Centre of Oil and Gas Resources, Ministry of Land and Resources, Beijing 100034, People’s Republic of China ‡
ABSTRACT: The Lower Cambrian shale is an important source rock for conventional oil and gas in northern Guizhou province (NGP), south China, but has recently been considered as a potential shale gas reservoir. In this study, characterization of this target shale was determined through a systematic series of measurements on core samples. According to total organic carbon (TOC) and Rock-Eval data, the Lower Cambrian shale is overmature but still contains high amounts of organic matter, which is composed mainly of alginite. The TOC content is enriched up to 10.4% at the base of the shale. There are two different correlation patterns between trace elements and TOC in the Lower Cambrian shale. A good correlation present in the upper part indicates a non-sulfidic anoxic sedimentary environment. However, higher enrichments of U, V, and Mo in the basal part exhibit sulfidic euxinic conditions. Brittle minerals are quite abundant and have a critical effect on artificial fracture. Gas sorption volume increases with an increasing TOC, pore volume, and surface area, indicating that pores at the microscale associated with the organic matter fraction are an important control on storage capacity. The contribution of clay minerals to the sorbed gas may be irrelevant because of the presence of moisture. Analysis of data indicates that the basal part of the Lower Cambrian shale demonstrates the greatest potential for gas content and can be explored for production.
1. INTRODUCTION With commercial production in the U.S.A., the shale gas boom is changing the world and has been the exploration focus for energy in many countries and regions (Canada, Europe, South Africa, and Australia) [U.S. Energy Information Administration (EIA); http://www.eia.gov/]. To boost national energy security, the Chinese government is also trying to develop this unconventional resource. Geological surveys and well drilling have increased recently. Strategic investigation by the Ministry of Land and Resources (MLR; http://www.mlr.gov.cn/ mlrenglish/) has revealed that shale gas resources could amount to 10.48 × 1012 m3 in Guizhou province alone. The Lower Cambrian shale in northern Guizhou province (NGP) of the upper Yangtze block (UYB) is not only rich in organic matter but also shows good gas concentrations and flow characteristics.1 However, few shale gas wells have been drilled in this area, and there is a lack of detailed data characterizing black shale in the literature. Researching the properties of shale gas reservoirs is very important to elucidate potential gas capacities while reducing exploration risk.2−4 Here, we report an investigation on the Cenye1 well, located in the Cengong area of NGP. This is a national geological investigation shale gas well with a completion depth of 1526 m. Hydrocarbon anomalies were monitored during drilling, and gas was produced after fracturing. The NGP area lies in the southeastern part of the UYB in south China. A disconformity occurred between the Upper Sinian and Lower Cambrian in the Late Proterozoic. During the Early Cambrian, a thick succession was deposited in an open marine platform to marine shelf environment, whose total original thickness could have been up to 1200 m.5,6 The generalized Lower Paleozoic stratigraphy is shown in Figure 1. The Lower © 2013 American Chemical Society
Cambrian succession in this region consists of black shale, carbonate rock, and siltstone and has been studied as a major source rock before.5,6 The organic-rich sedimentary rock sequence is developed widely throughout the NGP area and will be assessed as a potential shale gas reservoir. Because of sea level regression in the Middle to Late Cambrian, a predominantly restricted marine platform environment resulted in a thick carbonate succession. The succession dips gently at 5−12° in the Cenye1 area (Figure 1).
2. MATERIALS AND METHODS 2.1. Samples. A total of 31 core samples, including one Precambrian siliceous shale, were selected from the Cenye1 well according to intervals based on stratigraphic data and logging response. Sample distributions are shown in Figure 2 together with detailed core descriptions and well log curves. 2.2. Experimental Approach. In this study, the core samples were geochemically and petrophysically characterized (Table 1). To assess the shale gas significance of the investigated rocks, total organic carbon (TOC), trace element (TE) content [inductively coupled plasma−mass spectrometry (ICP−MS)], X-ray fluorescence (XRF), mineralogy [Xray diffraction (XRD)], Hg porosimetry, pore size distribution, and gas sorption capacity were determined and Rock-Eval pyrolysis was performed. The well logging data were collected from MLR. The TOC content was measured using a Leco SC-632 instrument, and a Rock-Eval 6 instrument was used for pyrolysis analysis, which provides Tmax, S1, S2, and S3. The temperature program was set as 300 °C for 3 min and then heating at 25 °C/min to 650 °C. Tmax is the required Received: January 24, 2013 Revised: May 7, 2013 Published: May 7, 2013 2933
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Figure 1. Studied locations, geological map of Cenye1 area, and stratigraphic column of the Lower Paleozoic in NGP. temperature for kerogen cracking.S1 is the amount of volatile hydrocarbon. S2 represents the amount of pyrolysis compound. S3 is the amount of CO2 generated.7 Al oxide was analyzed by XRF on dried powdered samples. TE (Sc, Zr, Th, U, V, and Mo) concentrations were determined by ICP−MS using sample powders dissolved in HF, HClO4, and HNO3. Bulk mineralogical compositions were measured with XRD using powdered rock samples. According to the International Union of Pure and Applied Chemistry (IUPAC) classification of porous materials, pores are divided into micropores (50 nm) based on diameter.8 The meso−macropore volume and surface area were detected using nitrogen (N2) gas adsorption. The methods of Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) were employed for calculation.9,10 Hg porosimetry was conducted on an Autopore IV 9510 series porosimeter. The pressure of Hg was increased up to 414 MPa, and mercury injection data were calculated for porosity (pore diameter > 3 nm).11 Methane sorption isotherms were determined on moistureequilibrated samples under reservoir temperatures. The Langmuir isotherm was used to model gas sorption capacity, V = VLP/(PL + P), where V is the volume of sorbed gas, VL is the Langmuir monolayer volume, P is the equilibrium pressure, and PL is the Langmuir pressure at which the sorption gas equals half of the Langmuir volume.12 The well log response was derived after drilling by the MLR, China.
3. RESULTS 3.1. TOC and Rock-Eval Pyrolysis. The TOC content and Rock-Eval data for 15 samples from Cenye1 well in NGP are presented in Table 2. TOC values vary from 0.73 to 10.40%, with an average of 4.35%. S1 and S2 values are extremely low in the Lower Cambrian shale (average of 0.06 and 0.11 mg/g, respectively). Anomalously low Tmax values are present in samples G012090 and CUGB22. Corresponding to the high Tmax values, thermal maturities are calculated above 3.0% (Ro = 0.0180Tmax − 7.16).13 Meanwhile, hydrogen indices (HI = S2/ TOC × 100) range between 1 and 4 mg of hydrocarbon (HC)/g of TOC, and oxygen indices (OI = S3/TOC × 100) vary from 3 to 38 mg of CO2/g of TOC. 3.2. TEs. The results for TE abundances and ratios (U/Al, V/ Al, and Mo/Al) in the Lower Cambrian shale of NGP are summarized in Table 3. The concentrations of redox-sensitive TEs (Al, Sc, Zr, Th, U, V, and Mo) show ranges from 1.7 to 13.3%, from 0.779 to 20.5 μg/g, from 12.6 to 260 μg/g, from 0.481 to 17.8 μg/g, from 3.97 to 97.3 μg/g, from 98.2 to 1682 μg/g, and from 2.55 to 269 μg/g, respectively. Meanwhile, U/Al, V/Al, and Mo/Al ratios vary from 0.33 to 13.70% (μg/g), from 10 to 236.9% (μg/g), and from 0.21 to 128.1% (μg/g), respectively (Table 3). The basal shale exhibits relative U, V, 2934
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Figure 2. Lithological column of Cenye1 well, sample locations, and log curves.
and Mo enrichments. Al versus Th, Zr, and Sc show positive correlations in the Lower Cambrian shale (Figure 3; R2 = 0.8548, 0.8921, and 0.9575, respectively). 3.3. Bulk Mineralogy. Mineralogical analysis (XRD) results are shown in Table 4. As the data indicate, the Lower Cambrian shale in NGP mainly contains clay and quartz minerals. Measured ranges of total clays are between 9 and 59% (average 34%), and quartz minerals are between 27 and 86% (average
49%). There is no more than 10% carbonate (calcite and dolomite) minerals (apart from two samples, CUGB11 with 32% and CUGB23 with 15%). Generally, illite is the dominant mineral in clays. There are minor amounts of pyrite and feldspar in the Lower Cambrian shale. Ternary plots of mineral composition are shown, so that a meaningful comparison can be made between the Lower Cambrian shale in NGP (Figure 4b) and hot shales with 2935
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comparison. The results show that the sorbed gas amount varied from 1.09 to 2.55 m3/ton, with a mean value of 1.79 m3/ton.
Table 1. Experimental Analysis of the Lower Cambrian Shale in NGP sample analysis
laboratory location
TOC and Rock-Eval pyrolysis TE content (XRF and ICP−MS) mineralogy (XRD) and mercury (Hg) injection nitrogen (N2) gas adsorption gas sorption capacity well logging data
4. DISCUSSION 4.1. Source Rock Characteristics. The Lower Cambrian shale evolved into the dry gas and thermogenic gas window, with organic matter being overmature (Ro ≫ 2%) in NGP. Although overmaturity was attained by shale rocks, the total organic carbon content is still relatively high. Sedimentation and organic carbon productivity rates are the controlling factors influencing the TOC concentration. With a changing depth of burial, both maturity and hydrocarbon generation levels were affected by geothermal heating. During the Late Jurassic, the geothermal temperature of the Lower Cambrian shale was close to 220 °C, which is significantly higher than present day temperatures, which range from 140 to 160 °C,14,15 and therefore, a complex thermal history can be inferred. It is noteworthy that there is no correlation between TOC and maturity of the Lower Cambrian shale in NGP, unlike the inverse relationship present in the Lower Cretaceous shale in northeastern British Columbia.16 Because of the high maturation levels, no distinct S2 peaks were present in Rock-Eval pyrolysis (Table 2); hence, erroneous Tmax values occurred with unreliable S2 values (less than 0.2 mg/g).17 The original kerogen type cannot be identified directly using a pseudo-Van Krevelen plot because of the extremely low hydrogen and oxygen indices. Previous studies demonstrated a sapropelic (type I) organic material composed mainly of alginite.6,18 This kind of organic matter has good to excellent generation capability before reaching overmaturity. Hunt19 suggested that the thermally mature fields may still potentially generate gas from the secondary cracking of hydrocarbon in situ. Jarvie et al.20 considered cracking gas to be the primary source for the Barnett shale in Newark East Field. Despite the fact that the Lower Cambrian black shale holds very low remaining hydrocarbon generation potential, large amounts of oil and gas must have been generated during its geological history. This case may show an insight for shale gas in NGP. 4.2. Redox Conditions. The redox classification of sedimentary settings is characterized by oxic, suboxic, and anoxic (including sulfidic and non-sulfidic) conditions. In response to the changing redox conditions, TE concentrations can be used as indicators of the paleoenvironment.21 Normalizing TE concen-
Applied Petroleum Technology AS, Norway CNNC Beijing Research Institute of Uranium Geology, China PetroChina, Huabei Oilfield Branch, China College of Chemistry and Chemical Engineering, China University of Petroleum, Beijing, China Research Institute of Petroleum Exploration and Development, Langfang Branch, China MLR, China
successful exploitation in the U.S.A. (Figure 4a). According to the diagram, two distribution regions can be recognized in Figure 4a. For the Bossier shale, the content of clay minerals is below 70%, total quartz, feldspar, and pyrite are lower than 50%, and carbonate varies between 5 and 90%. In the other region, quartz, feldspar, and pyrite minerals range from 25 to 80%, the clay content is between 15 and 65%, and carbonate is lower than 25% in the Ohio and Woodford/Barnett shales (west Texas). For the siliceous Barnett shale, total quartz, feldspar, and pyrite are higher than 40% and clays are no more than 50%. As the comparison indicates (Figure 4b), the mineral composition of the Lower Cambrian shale in NGP is similar to the Ohio and Woodford/Barnett shales (west Texas). 3.4. Porosity. We studied the porosity of the Lower Cambrian shale in NGP through mercury (Hg) injection and nitrogen (N2) gas adsorption determinations. As seen in Table 5, all samples apart from CUGB22 exhibit low values of Hg porosity (
