Available online at www.sciencedirect.com ScienceDirect Natural Gas Industry B 5 (2018) 132e138 www.elsevier.com/locate/ngib Research Article Application of element logging to lithologic identification of key horizons in SichuaneChongqing gas provinces*,** Yin Ping a, Qi Lin a,*, Zhu Qianxia a, Luo Li a, Tang Jiaqiong a, Ruan Cong a, Zhao Lei a & Liang Hong b a Geology Exploration and Development Research Institute, CNPC Chuanqing Drilling Engineering Co., Ltd., Chengdu, Sichuan 610051, China b No.4 Gas Production Plant, PetroChina Changqing Oilfield Company, Yinchuan, Ningxia 750006, China Received 23 March 2017; accepted 25 September 2017 Available online 21 March 2018 Abstract With the popularization and application of fast drilling technology in SichuaneChongqing gas provinces, the returned cuttings are fine and even powdered, so the traditional cutting lithology identification methods are not applicable any longer. In this paper, qualitative lithology identification and quantitative interpretation based on element logging were conducted on the key oil and gas bearing layers in this area according to the principle of elemental geochemistry. The study results show that: (1) different lithologies can be identified easily because of their different element logging characteristics. For example, basalts have the element characteristics of 0.35 < Fe/Si < 0.55 and Ca/(Na þ K) <4.00, while sedimentary rocks have Fe/Si < 0.35 or >0.55. (2) Clastic rocks, carbonate rocks, sulphate and transition rocks in the category of sedimentary rocks can be identified based on the element combination of (Al þ Si þ Fe) and (Ca þ Mg þ S). Among them, clastic rocks have (Al þ Si þ Fe) >31%, carbonate rocks have (Al þ Si þ Fe) <8% and (Ca þ Mg þ S) <36%, and sulphate rocks have (Al þ Si þ Fe) <5% and (Ca þ Mg þ S) >36%. (3) Then, based on the element combination of (Si þ K þ Ca)þSi/Al and (Al þ Si þ Fe þ K)þ(Ca þ S)/Mg, sandstone, mudstone (shale), gypsum, dolomite, limestone and transition rocks can be identified. Finally, a qualitative identification chart and a set of quantitative interpretation software of element logging on key horizons in SichuaneChongqing gas provinces were developed to make this method convenient for field application. This method was applied on site in ten wells (such as Well MX207) in SichuaneChongqing gas provinces. It is indicated that the coincidence rate of lithology identification is in the range of 88.75e95.22% (averaging 92.42%). Obviously, it can satisfy the requirements of fast lithology identification while drilling of key oil and gas horizons in SichuaneChongqing gas provinces. © 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: SichuaneChongqing gas provinces; Key horizon; Element logging; Lithologic identification while drilling; Quantitative lithologic interpretation; Qualitative identification chart; Interpretation software Along with the continuous progress of oil and gas exploration and development in SichuaneChongqing gas provinces, and with the popularization and application of fast drilling technologies (such as PDC drill bit þ stud and gas drilling), the returned cuttings are fine and even powdery, rock textures are seriously destroyed, so the traditional cuttings lithology identification methods (such as by naked eyes and by microscope) are not applicable any longer, which has directly affected the accuracy of key horizon calibration and even engineering progress. Element logging can effectively realize * Project supported by the Science and Technology Project of CNPC Chuanqing Drilling Engineering Co., Ltd “Element logging features of key horizons and application in SichuaneChongqing gas provinces” (No.: CQ2014B-14-1-1). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn.10000976.2017.09.004. * Corresponding author. E-mail address: firstname.lastname@example.org (Qi L.). Peer review under responsibility of Sichuan Petroleum Administration. https://doi.org/10.1016/j.ngib.2018.01.005 2352-8540/© 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Yin P. et al. / Natural Gas Industry B 5 (2018) 132e138 fast while-drilling lithologic identification of fine-powered cuttings and horizon judgment . Element logging is a type of mud logging technology that analyzes element content in cores and cuttings by means of Xway luminoscope, and identify lithologies based on the element content [2e5]. This technology is to identify lithologies and evaluate formations by measuring element content in stratigraphic rocks and then by analyzing the variations of rock elements and element combination features . It is not subject to the influences of well bores, drilling fluid properties, drilling modes and personnel experiences, etc. After years of development, element logging technology has been favorably applied in identifying lithologies of clastic rocks, carbonate rocks, volcanic and metamorphic rocks, in stratigraphic division and correlation, and in deposit facies analysis etc. in some oil fields (Changqing, Tarim, Bohai Bay, Liaohe, Daqing, Yuanba, Fuling and Tuha, etc.) [7e13]. In this study, element logging measurement was conducted to more than 1.2 104 cuttings samples from four wells (Well YA24 in the east, Well MX105 in the middle, Well G2 in the south and Well HS1 in the west of the Sichuan Basin). Besides, normalization and statistical analysis was conducted to the measured data. Finally, the lithology identification method was established based on element logging for the key horizons in SichuaneChongqing gas provinces, so as to fast identify the lithologies of key horizons in the process of drilling. 1. Confirmation of key horizons in SichuaneChongqing gas provinces During Sinian PeriodeMiddle Triassic Period, the Sichuan Basin was mainly in marine facies, receiving the deposits of carbonate rocks interbedded with muddy shale or other clastic rocks. After Late Triassic Period, it was chiefly continental deposits with lithologies of terrigenous clastic rocks interbedded with carbonate rocks in lacustrine facies [14e16]. As the depositional environments in various geologic periods 133 varied a lot, and the erosion degrees in various areas were different as the result of tectonic movements, and the contact formations beneath and over the weathered and eroded interfaces are also different somewhat. Based on current petroleum exploration and development status in SichuaneChongqing gas provinces, the calibration horizons in cementation of intermediate casing and production casing, primary drilling target horizons and secondary drilling target horizons were confirmed as the key horizons. Exploration and development practices show that the key horizons in SichuaneChongqing gas provinces mainly include Upper Triassic Xujiahe Fm (T3x), the 1st and 4th Members of the Middle Triassic Leikoupo Fm (T2l1and T2l4), the 2nd Member of the Lower Triassic Jialingjiang Fm (T1j2) and the Feixianguan Fm (T1f ), the Upper Permian Changxing Fm (P3c), the Middle Permian Maokou Fm (P2m), the Lower Permian Qixia Fm (P1q), the Carboniferous System (C), the Lower Cambrian Longwangmiao Fm (V1l ), and the Sinian Dengying Fm(Sd ), etc. (Table 1). 2. Lithologic identification based on element logging 2.1. Qualitative identification Studies on geochemistry show that rocks are composed of the oxides of some major elements (C, H, O, Na, Mg, Al, Si, S, Ca, K, Fe, Mn etc.), and various lithologies have different content of the major elements . The average oxide content of these elements is 98.3% in acidic rocks, 97.7% in intermediate rocks, 95.9% in basic and ultrabasic rocks, 94.9% in sandstones, 91.4% in mudstones, and 60% in carbonate rocks . Because of the complexity and the variety of deposition, the element compositions of deposit rocks have more substantial variations. In clastic formations, the main characteristic elements are Si and Fe (among them, Si represents sandy materials, Fe represents muddy materials, and Ca indicates certain contents of limy materials), while coals feature low Si Table 1 Key horizons and features of their major lithologies in SichuaneChongqing gas provinces. Region Cementing horizon of Lithologic feature intermediate casing Eastern Sichuan Top of T1j32 Central Sichuan Top of Tx Top of T1j32 Southern Sichuan Top of T3x Western Sichuan Top of T3x Cementing horizon of Lithologic feature production casing Gypsum interbedded Top of with dolomite Top of Top of Sandstone Top of Gypsum interbedded Top of with dolomite Top of Top of Sandstone Top of Top of Sandstone Top of Top of Top of T1f3 P3c C T3x4 T2l V1l Sd P2m P1q2 T3x2 T3x1 T2l Top of P2m Top of Sd Target horizon Lithologic feature Limestone Limestone Limestone or dolomite Sandstone Dolomite or argillaceous limestone, muddy dolomite Sandy dolomite Dolomite Limestone Limestone Sandstone Sandy limestone Dolomite or muddy dolomite T1f P3c C T3x4 T2l1 and T2l4 Oolitic dolomite Dolomite Dolomite of limestone Sandstone Pinhole-like dolomite V1l Sd P2m P1q T3x2 T3x1 T2l Limestone Dolomite P2m and P1q Sd Dolomite Dolomite Limestone Limestone Sandstone Sandstone Dolomite interbedded with limestone and gypsum Limestone and dolomite Dolomite 134 Yin P. et al. / Natural Gas Industry B 5 (2018) 132e138 Fig. 1. Features of element logging curves. content, moderate Fe content and high S content (Fig. 1-a). In carbonate formations, the main characteristic elements are Ca, Mg and S (among them, Ca represents limy materials, Mg represents dolomitic materials, and S represents gypsum materials) (Fig. 1-b). Lithologies can thus be identified according to the variations of elements and their combinations. According to the statistics of element logging data of cuttings from four typical wells (Table 2 and Table 3), the Fe/Si and Ca/(Na þ K) element combinations of magmatic rocks (basalts) and deposit rocks in SichuaneChongqing gas provinces are apparently different: in basalts, Fe/Si is greater than 0.35 and smaller than 0.55 and Ca/(Na þ K) is smaller than 4.00; in deposit rocks, Fe/Si is smaller than 0.35 or greater than 0.55. Based on the above element combination and variation features, the lithologic identification chart of magmatic and sedimentary rocks in SichuaneChongqing gas provinces was established (Fig. 2). Sedimentary rock is the major rock type in key horizons in SichuaneChongqing gas provinces, which includes clastic rock, carbonate rock, sulphate rock and transition rock. In clastic rocks, Al, Si and Fe are the major composition elements; in carbonate rocks, Ca and Mg are the major composition elements; in evaporite rocks, Ca, S, Na, Cl and K are the major elements. Affected by drilling fluids, halite and halosylvite are hard to be preserved in cuttings, thus the major composition elements are Ca and S in evaporite rocks. In transition rocks, as they contain different minerals, their major composition elements are somewhat different. According to further analysis of the data in Table 2, 25% of the (Al þ Si þ Fe) content (mass fraction, similarly hereinafter) is the boundary of clastic rocks, carbonate and sulphate rocks: the value of clastic transition rocks is between 25% and 31%; the value of clastic rocks is higher 31%; the value of carbonate transition rocks is between 8% and 25%; the value of Table 2 Statistics of percentages of characteristic element combinations of major rock types in key horizons in SichuaneChongqing gas provinces. Lithology Si þ K þ Ca Basalt Argillaceous sandstone Sandstone Lime sandstone Mudstone Shale Carbonaceous shale Limy shale Argillaceous limestone Limestone Dolomite Sandy dolomite Gypsum 28.72e32.45% 24.86e33.89% 32.49e40.60% 30.00e38.05% 24.71e35.36% 16.07e31.06% 32.60e33.94% 28.52e29.24% 34.15e36.88% 33.08e37.12% 22.54e27.45% 26.79e31.45% 16.24e17.20% (30.89%) (26.85%) (36.65%) (35.77%) (28.55%) (25.26%) (33.34%) (28.88%) (36.00%) (35.57%) (24.25%) (27.92%) (16.62%) Note: The data in brackets are average values. Al þ Fe Al þ Si þ Fe þ K Ca þ Mg þ S Al þ Si þ Fe 14.26e17.60% (16.56%) 13.52e17.86% (16.88%) 6.49e15.40% (10.47%) 5.38e14.38% (7.55%) 9.17e19.23% (15.68%) 12.88e29.81% (18.98%) 9.79e10.87% (10.36%) 9.43e11.39% (10.41%) 2.06e5.38% (2.69%) 0.59e1.67% (1.00%) 0.17e3.61% (0.69%) 0.16e5.15% (1.13%) 0.08e0.35% (0.16%) 38.20e40.92% (39.82%) 39.35e47.33% (41.45%) 40.59e50.22% (45.76%) 22.87e36.10% (29.08%) 36.58e44.77% (41.51%) 33.85e45.86% (39.49%) 37.33e40.41% (39.66%) 26.41e27.18% (26.80%) 9.51e21.15% (11.88%) 3.86e9.53% (6.34%) 2.15e10.89% (3.69%) 8.85e25.80% (12.33%) 0.75e1.67% (1.16%) 9.56e12.80% (10.57%) 0.81e5.79% (3.88%) 0.81e6.07% (2.24%) 10.13e20.62% (15.54%) 1.18e8.94% (4.30%) 0.93e11.60% (6.84%) 6.57e9.19% (7.20%) 16.27e16.78% (16.53%) 21.44e30.98% (28.96%) 30.81e35.07% (33.43%) 28.31e33.80% (32.55%) 18.07e28.68% (26.48%) 39.53e41.63% (40.92%) 36.52e39.28% (38.34%) 36.2e44.41% (38.11%) 37.91e47.47% (43.22%) 20.89e32.44% (26.84%) 31.93e40.79% (37.93%) 31.85e42.98% (37.53%) 34.33e37.48% (36.77%) 25.64e26.79% (26.22%) 7.99e19.23% (10.31%) 2.92e7.23% (4.70%) 2.01e8.95% (3.52%) 8.80e21.61% (11.47%) 0.72e1.52% (1.10%) Yin P. et al. / Natural Gas Industry B 5 (2018) 132e138 135 Table 3 Statistics of ratios of characteristic element combinations of major rock types in key horizons in SichuaneChongqing gas provinces. Lithology Fe/Si Basalt Argillaceous sandstone Sandstone Lime sandstone Mudstone Shale Carbonaceous shale Limy shale Argillaceous limestone Limestone Dolomite Sandy dolomite Gypsum 0.37e0.52 0.09e0.26 0.05e0.35 0.12e0.30 0.10e0.27 0.13e2.37 0.11e0.14 0.39e0.56 0.10e0.14 0.07e0.14 0.01e1.40 0.01e0.20 0.03e0.20 (0.47) (0.22) (0.11) (0.17) (0.20) (0.65) (0.13) (0.47) (0.13) (0.11) (0.25) (0.04) (0.07) Ca/(Na þ K) (P þ S)/Cl (Ca þ S)/Mg Si/Al 1.05e2.72 (1.72) 0.02e0.79 (0.45) 0.02e1.17 (0.28) 1.51e6.09 (3.86) 0.02e1.55 (0.53) 0.03e3.30 (1.52) 0.62e1.16 (0.79) 4.25e7.21 (5.61) 5.82e13.44 (10.04) 7.28e15.72 (11.07) 4.54e15.67 (9.93) 1.82e11.20 (7.39) 6.90e9.66 (8.16) 0.88e4.17 (2.17) 0.50e41.96 (13.28) 0.95e16.22 (3.73) 1.39e9.92 (4.08) 0.35e73.48 (8.32) 1.60e59.98 (12.76) 89.64e261.17 (156.72) 13.61e20.19 (16.65) 5.43e53.85 (12.94) 5.93e68.53 (20.04) 9.38e81.58 (19.00) 13.88e29.57 (20.73) 93.61e247.98 (160.16) 1.74e5.21 (3.50) 0.19e2.87 (1.46) 0.26e11.78 (2.61) 4.61e21.87 (14.34) 0.12e7.95 (2.16) 0.23e10.10 (5.11) 2.21e4.10 (3.14) 13.45e18.59 (15.88) 12.12e32.58 (22.30) 8.57e46.64 (17.35) 1.74e2.39 (1.97) 1.69e2.76 (1.88) 12.88e88.27 (44.72) 2.90e4.32 (3.44) 1.47e2.84 (1.71) 2.84e7.62 (5.18) 2.01e8.09 (5.12) 1.43e4.81 (2.17) 1.22e6.64 (2.81) 3.55e4.06 (3.79) 5.14e5.44 (5.29) 3.67e5.14 (4.52) 3.49e10.45 (6.53) 5.51e86.30 (22.24) 4.38e93.53 (59.76) 8.54e16.11 (11.46) Note: The data in brackets are average values. transition rocks is lower than 8% and (Ca þ Mg þ S) is lower than 36%; the value of sulphate rocks is less than 5% and (Ca þ Mg þ S) is more than 36% (Fig. 3). Though the dolomite, limestone and transition rocks in carbonate rocks can be better separated and they have distinctive linear relationship, clastic rocks have no such a relationship, but present a chaotic distribution and poor separating degree. According to statistics, it is also indicated that sandstone and its transition rocks in clastic rocks have (Si þ K þ Ca) content higher than 32% and Si/Al greater than 4; while mudstone (shale) and its transition rocks have (Si þ K þ Ca) contents lower than 32% and Si/Al smaller than 4. Among them, carbonaceous shale has a high (P þ S)/Cl ratio (>80), argillaceous sandstone has a low Si/Al ratio (1.7) and a high Al þ Fe content (16.88%), and limy shale has a high Si/Al ratio (5.28) and a moderateelow Al þ Fe content (10.41%). Lithologic identification chart of clastic rocks based on element logging was established accordingly (Fig. 4). Carbonate and sulphate rocks are widely distributed in Triassic System and underlying formations in SichuaneChongqing gas provinces. Because of quick lithofacies change, there are more transition rocks. Studies indicate that argillaceous limestoneesandy dolomiteelimestonee dolomiteegypsum have decreasing (Al þ Si þ Fe þ K) content, and gypsumeargillaceous limestoneelimestonee dolomiteesandy dolomite have decreasing (Ca þ S)/Mg values. Accordingly, the lithologic identification chart of carbonate and sulphate rocks was established based on the relationship of (Al þ Si þ Fe þ K) and (Ca þ S)/Mg (Fig. 5). The (Al þ Si þ Fe þ K) content of gypsum is lower than 2%; the (Al þ Si þ Fe þ K) content of dolomite is between 2% and 8.5% and the (Ca þ S)/Mg ratio is smaller than 6; the (Al þ Si þ Fe þ K) content of limestone is between 2% and 8.5% and the (Ca þ S)/Mg ratio is greater than 6; the (Al þ Si þ Fe þ K) content of sandy or argillaceous dolomite is higher than 8.5% and the (Ca þ S)/Mg ratio is smaller than 6; the (Al þ Si þ Fe þ K) content of argillaceous limestone is higher than 8.5% and the (Ca þ S)/Mg ratio is greater than 6. Fig. 2. Lithologic identification chart of magmatic and sedimentary rocks based on element logging in SichuaneChongqing gas provinces. Fig. 3. Lithologic identification chart of sedimentary rocks by element logging in SichuaneChongqing gas provinces. 2.2. Quantitative interpretation The key horizons in SichuaneChongqing gas provinces have diversified types of lithologies. Based on charts, the rock types of samples can be qualitatively confirmed. However, 136 Yin P. et al. / Natural Gas Industry B 5 (2018) 132e138 Fig. 4. Lithologic identification chart of sandstone, mudstone (shale) based on element logging. transition rocks (such as sandy shale/mudstone and argillaceous/shaly sandstoneargillaceous sandstone, limy dolomite and dolomitic limestone, sandy dolomite and dolomitic sandstone) have to be quantitatively interpreted and accurately named through processing and calculation of element logging data. Due to the limitation of the measuring apparatus, C, H and O cannot be measured. Thus, for coaly and carbonaceous shale with rich C, H and O element, the total element content is apparently lower: generally, the total element content of coal is lower than 30%, and that of carbonaceous shale is between 30% and 40%. Therefore, coal and carbonaceous shale can be identified firstly. Accordingly, normalization is conducted to the measured data, followed by qualitative lithologic identification to the results by charts, to quantitatively calculate element content of secondary compositions in rocks. For clastic rocks, sandstone and mudstone content are allocated as per Si/Al ¼ 4, to calculate sandstone content and mudstone content respectively; for carbonate and sulphate rocks, allocation is conducted according to the reverse deposit sequence by chemical method, followed by the calculation of gypseous, dolomitic and limy content, and by the interpretation of the lithologies of the samples according to rock nomenclature. A set of software was developed for quantitative lithology interpretation based on element logging to facilitate field application. Through on-site experiments, the comparison between quantitative lithology interpretation profile based on element logging and well logging interpretation profile (Table 4) shows that the coincidence rates are between 88.75% and 95.22% (averaging 92.42%), meeting the requirement of on-site mud logging. 3. Application cases Fig. 5. Lithologic identification chart of carbonate and sulphate rocks based on element logging. Well MX207 is an appraisal well in central Sichuan Basin, and its drilling target is the Longwangmiao Fm that gradationally contact with overlying formations. The PDC drill cuttings are tiny, thus they are hard to be distinguished. Element logging shows that at the well depth of 4676.00 m, Ca Table 4 Statistics of coincidence rates in lithologic identification in key horizons based on element logging in SichuaneChongqing gas provinces. Well name Identified horizon Total thickness of identified lithologies*/m Dolomite Limestone Gypsum Shale Sandy Mudstone Dolomitic Siliceous Bauxitic dolomite silt dolomite dolomite T1j3eT1j32,V1qeSd3 T1j3eT1j22, V1q, Sd4eSd3 T1j3eT1j32, V2xeV1c, V1qe Sd4, Sd4eSd3 MX111 T1j3eT1j1,V2geV1c, V1qeSd4 MX119 T1j3eT1j1, V2xeV1c, V1qeSd4 MX207 T1j3eT1j32, V2xeV1c HT1 T1j3eT1j1, O1teV1l, Sd4eSd2 NC2 T1j3eT1j32, P2leP1q MX001eH6 T1j32eC SF1 T2leS GS16 MX39 MX41 76/79 100/100 118/115 45/41 45/39 27/25 25/24 58/57 35/39 50/54 17/15 12/18 15/16 22/23 82/71 251/261 462/468 168/165 575/578 18/21 37/31 310/339 108/99 111/102 4/5 105/101 408/410 254/252 1704/1702 66/60 35/36 24/26 115/112 36/31 83/89 497/478 40/42 18/19 41/42 18/23 3/4 42/47 9/9 13/15 43/45 92.83% 90.89% 91.38% 37/37 69/80 42/44 73/69 57/56 32/37 11/11 100/91 Note: The total identified thickness like “76/79” means that based on element logging vs. well logging. Coincidence rate 49/45 8/8 9/8 94.03% 93.04% 88.75% 93.52% 95.22% 90.73% 93.82% Yin P. et al. / Natural Gas Industry B 5 (2018) 132e138 137 Fig. 6. Comparison of lithologic interpretation results between element logging and well logging at the interval of 4640.00e4850.00 m in Well MX207. content is 27.25% Y 13.06%, Mg content is 13.06% Y 8.17%, Al content is 0.66% [ 3.89%, and Si content is 4.56% [ 18.60% with apparent increase, thus the identified lithology is sandy dolomite, which enters the Gaotai Fm. At the well depth of 4732.00 m, the content of Mg and Ca, Al and Si present four times of alternative increase and decrease. According to element logging interpretation, there are three sets of sandy dolomite interbedded with two sets of dolomite. At the well depth of 4733 m, Ca content is 10.81% [ 24.37%, Mg content is 4.88% [ 11.26%, Al content is 2.03% Y 0.59%, and Si content is 14.06% Y 3.58%. According to element logging interpretation, it is dolomite, which enters the Longwangmiao Fm. At the well depth of 4830.00 m, Ca content is 25.15% Y 16.97%, Mg content is 12.76% Y 9.06%, Si content is 3.55% [ 13.18%, Al content is 0.75% [ 4.10%. According to the element logging interpretation, it is sandy dolomite, which enters the Canglangpu Fm. These lithologic interpretation results based on element logging are consistent with that based on well logging (Fig. 6). 4. Conclusions 1) Element logging technology can provide plentiful element analysis data of stratigraphic rocks. It has special advantages in identifying lithologies, and can effectively deal with the difficulty in lithologic identification caused by the development of drilling techniques and depositional environments etc. 2) As a type of special mud logging technique, it can quickly and accurately identify lithologies while drilling, and provide reliable geologic data and a technical support for confirming and classifying important stratigraphic interfaces and key horizons, as well as for drilling engineering. 3) The field application results of ten wells (Well MX207 etc.) in SichuaneChongqing gas provinces show that the coincidence rates of the identified lithologies based on element logging technology are between 88.75% and 95.22% (averaging 92.42%). 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