Study on the threshold value of organic enrichment of carbonate as gas source rocks

Science in China Ser. D Earth Sciences 2004 Vol47 Supp. I 150-158Study on the threshold value of organic enrichment ofcarbonate as gas source rocksXUE Haitao, LU Shuangfang, ZHONG Ningning WANG Bo'1. Daqing Petroleum nstitute, Daqing 163318, China;2. University of Petroleum, Beijing 102200, ChinaCorrespondenceshouldbeaddressedtoXueHaitao(email:geochem@dapi.net)Received October 17, 2003; revised August 12, 2004Abstract In this paper, calculations have been performed about gas quantity of generationadsorption, dissolving in oil, dissolving in water, diffusion of unit area carbonate rocks at differentgeologic conditions in the Tarim basin. According to the material balance principle, the corresponding organic carbon content when gas started expelling from source rocks with separatephases has been worked out. We regard it as the theoretical threshold value(TOCmin) of gassource rocks under the same geologic condition. Based on the simulating calculation, a fact hasbeen discovered that ToCmin decreases with the increasing source rocks thickness, decreases atthe beginning and then increases with the increasing maturity and decreases with the better typeof organic matter. TOCmin evaluation table of carbonate gas source rocks in the Tarim basin hasbeen established. Investigations indicate that the TOCmin of carbonate gas source rocks variesgreatly with the differences of geologic conditions and gas source rocks cannot be evaluatedwith a unified ToC threshold value. And we also establish a preliminary evaluation table of Tocindustrial threshold value, TOCa, of carbonate gas source rocks in the Tarim basinKeywords: carbonate rocks, gas source rocks, threshold value of organic enrichment, material balance principle, total organic car.(TOC).DOI:10.1360/04zd0034The oil and the gas reserves of carbonate rocks carbonate rocks sediment strata ofaccount for 40% and 28 of the world,s respectivelChina, distribution of hydrocarbon source rocks is notIn China. the distribution area of carbonate rocks is 3 only thick, but also broad, which is not incompatiblemillion km 2 or so, which occupies nearly 1/3 of the with the results from exploration practice apparen-whole land, and the features of carbonatetlylarge deposition thickness, poor organic matter andExploration practice indicates that the content ofhigh maturity. Therefore the determination of therganic matter in most carbonate rocks is underthreshold value of organic enrichment becomes the 0.4%-0.5% which is used as the threshold value offocus in the evaluation of carbonate source rock. Be- organic enrichment( 3. 5] of oil source rocks, in Chinafore the middle period of 1990s, the threshold value of But whethe中国煤化工 e rocks, due toorganic enrichment proponed by most of Chinese the low abulCNMH Gome oil sourcescholars[11(0.1%-02%)is obviously lower than the rocks can be gas source rocKs or not Is the great prob-international standard. Upon the threshold value, in the lem explorationists are concerned about. If we useCopyright by Science in China Press 2004Study on the threshold value of organic enrichment of carbogas source rocks151different threshold values of organic enrichment, there experiment, therefore, in this paper, the experimentwill be many differences to confirm thickness and carried out on low maturity ,=0.37%)and highdistribution of gas source rocks, which will have direct enrichment TOC (TOC = 4.49%)Santanghu Permianinfluences on the resource evaluation 6l as a resultcarbonate samples which are type II kerogen and con-the study of evaluation criterion of carbonate gas sour- tain 73.75% carbonate mineral. Pyrolysis hydrocarbonce rocks is a basic research subject which has signifigeneration experiments are made on Rock-Eval-IIcant influences on exploratory decision. It has imporapparatus under the heating rate conditions of 9.61tant theoretical and practical value/min, 19.2C/min and 408T/min respectively. BCalculations have been performed aboutusing PY-GC, thecarbon(oil gas) generatingquantity of generation, adsorption, dissolving in oil, ratio-temperature curve is transformed into oildissolving in water, diffusion of unit area carbonate generating ratio-temperature and gas generating ratiorocks at different geologic conditions in the Tarim terature curve for calibrating oil and gasbasin. According to the material balance principle, the generating kinetic modelscorresponding organic carbon content when gas start-ed expelling from source rocks with separate phasesDehydrated oil samples of the Tarim basin are puthas been worked out. We regard it as the theoretical into thin wall(0. 2 mm) golden tubes which is weldedthreshold value(TOCmin) of gas source rocks under theby high frequency welding machine after substitutingcologic condition The main effective facair with Ar. Then the welded golden tubes are placedthreshold value of organic enrichment of carbonate gasin autoclaves in which water is used as pressure medi-source rocks have been discussed. The industrial um and autoclaves are laid in constant temperaturethreshold value of organic enrichment (toCe of carwater tank which can control heating rate. This systembonate gas source rocks has been calculated in accorcan hold 15 autoclaves at the same time. Under thedance with the lower limit standard of gas generationconditions of 60 MPa pressure, the system is heated upfrom200℃to600℃ at the heating rate of20℃/hAtstrength of industrial reservoira certain target temperature, a golden tube is taken out1 Experimentsand generating gas is measured with a gas collectionSince Tissot introduced the chemical kinetics and quantification system. Analysis of gas compos itheory into the study of hydrocarbon generation from tion is made by HP6890 gas chromatograph and the oilsource rocks [7l, there have been many scholars, at to gas conversion rate-temperature curves are obtained,home and abroad. who have researched the kinetics of which are used for calibrating oil to gas kinetic modelhydrocarbon generation from organic matter and itsIn order to set up adsorbed gas model and disapplication problem/8-l0, which makes it, togetherlved gas in oil model, the authors carry out sourewith the thermal simulation experiment method, be- rock adsorbing gas experiments[2l and gas dissolvincome the main method of evaluation of hydrocarbon in oil experiments)generation of source rocks l. This paper uses chemi-al kinetics method for its strong theoretical basis, and 2Modelsin order to obtain the hydrocarbon generating ratio- 2. 1 Computational model of the amount of gas gen-temperature curve for calibrating kinetic models, des- erationout oil and gas generating from or-ganic matter and oil to gas pyrolysis experimentsBased on the data derived from experiments ofoil and gas generation from organic matter and oil toIn the Tarim basin, there are no immature sam- gas, the cor中国煤化工 eters are caliples which are fit for hydrocarbon generating pyrolysis brated(seeHiled process ofCNMHG1)Xue Haitao, Study of carbonate source rocks evaluation criterion, Ph. D dissertation (in Chinese), Daqing Petroleum Institute, 2003Science in China ser d Earth SciencesTable I Kinetic parameters of oil and gas generating from organic matter in Santanghu calcareous rockOil generating from organic matterGas generating from organic matterAOriginal poteOriginal potential160873X10984×103961×10649X101.370×10126.16×1083.058×1031.70×10-433.73×1083015×102791×10162.026X1.33×1022977×102538×10-139455×10149,57X106.301×103213.75X10220141X1024082×10169.03X103.524X8.60×1059×1093.08×101.156×10154.65×1064.093×10151.122X102.10X131X10682×1042804.40×1062.347X10L.026X104.49X103.07X101.007X10137.22X1081070×10142.45×10-121.002×1031.96×1031.016×10141.28×10-10242×1071004×1041.000×1013567X106001X101.32×10-0000X106.32×101000×10144.36X10Table 2 Kinetic parameters of oil to gas in the Tarim basinActivation energPre-exponentialActivation energyOriginal1.000X10123.47×10-172404.681×10121.14X10899×10481.83X10-01.000×10123.92X102.754X10104.08X1.124×1091.07X1038X107488X6.34×10+719001×10941X101.62×10-46.082×1019,46X109899X103.68X2107.30X10-12972X10011X10-169993×1014.865×1048.60×10+calibration has been reported by Lu Shuangfang)In this way, total gas-generating and net oil-genUsing the above kinetic parameters and combin-erating can be calculated quantitatively and dynami-ing with the sedimentary-burial and geothermal histocally. Of which, net oil-generating calculation bery of source rocks in target zone, we can calculate comes the basis of calculation of the amount of disquantitatively the yield of oil and gas from kerogensolved gas in oilprimary cracking, and the yield of gas from oil secon- 2.2 Computational model of adsorbance of gasdary cracking at any time, and then calculate net oilSource rocks adsorption is an important form forgenerating and total gas-generating as followsa series of strata to remain oil and gas (including sourNet oil-generating= oil generated from kerogen ce rocksninus oil cracking to gas中国煤化工CNMHG principle, theTotal gas-generating gas generated from kero- authors design a set or experimental equipment forgen add gas generating from oilmeasuring adsorbance of natural gas in rock, and conStudy on the threshold value of organic enrichment of carbogas source rocksstruct and calibrate adsorption models as follows 2formula. Combining with the relationship of porosityand depth of all rocks, we can figure out the amount ofa·P(1) dissolved gas in water in any volume(or weight)rockIn this paper, solubility experiment of natural gasA-+B-+C(2) in oil has been carried out and the dissolving law ofnatural gas in crude oil has been obtained, in which theX·-+Y,(3) solubility increases with the increasing pressure, anddecreases with the increasing temperature, and solubilitational model of gas in crude oil has beer(mol/kg)ofum adsorbance ofadsorptionP(MPa) is gas phase pressure, T(K) is system tem-K(D pxr doperature, A, B, C, D, x, r are constant parametersMrf(t)+at)k(OpMrf(t)+k(tpxrdoftvwhich can be obtained through calibrationwhere C is the solubility(mol/cm)of gas in crude oil,Based on the mentioned models, using weighted Mr is the average molecular weight of hydrocarbon, tsumming method, adsorbance of gas in source rocks is the system temperature(t), is the second reaand wall rocks which have main lithologic association tion factor, K is the solubility constant, xr is the weightand organic matter abundance can be calculated:fraction of hydrocarbon, p is the gas partial pressure∑nf(Pa),fperature coefficient, do is the den(g/cm) of ground crude oil,Vnt liquid fawhere f i is the percentage of some lithology or kerogenmolar volume of gas(cm/mol)accounts for, n is the corresponding adsorbance ofCombining with the calculated net oil-generatinggas.of source rocks we can figure out the amount of dis2.3 Computational models of the amount of dis- solved gas in oil quantitatively through the above forsolved gas in water and oilmulPores of source rocks and wall rocks in a series of 2.4 Computational model of the quantity of gas difstrata are full of formation water and generated crude fusionoil. And oil and water can dissolve natural gas, whichThe rate of diffusion of natural gas is very slocannot be expelled out effectively. According to the in underground rocks. But during the long geologicalinvestigations, natural gas solubility in oil is higher period in history, the accumulative diffusion flux is sothan that in water. But the volume of formation water large that it can destroy a natural gas reservoir whichin the whole source rocks series of strata is very large, is worth exploiting in industryll5l. On the basis of thethen the amount of dissolved gas in water cannot be established models by others[6, 17, the quantity of natural gas diffusion in target zone under differentBy the study of dissolving mechanism of gas in geologic conditions is calculated, and the specificpure water and brine, Fu Xiaotai et al. 3, 4 have estab- model is the following:lished the universal and applicable solubility formulaQ=D A6)to describe the quantity of water dissolving in gas,hich can be figured out at any temperature, salt con- where Qisal gas diffusiontent and under any pressure conditions through this (m), DisTHEn MES- ient(m2/s),A1)See footnote 1)on page 151Science in China ser d Earth Sciencesthe spreading area of natural gas diffusion(m), t is the source rocks thickness, Hy. For instance, when HInatural gas diffusion time(s), dC/dx is the natural gas equals 500 mgHC/gC, Zdm equals 5000 m, the threshconcentration gradient(m /m/m)old value of organic enrichment decreases from 0.273 The threshold value of organic enrichment ofto 0. 15 with the increasing thickness(fig. 1). Thecarbonate gas source rockthreshold value has great variance with the decreasingthickness. When thickness increases from 50 m to 200Using the parameters of geohistory and thermal m. toc decreases from 0.27 to 0.25: but when thick-history in the Tarim basin and strata of well Tazhong ness increases from 3000 m to 5000 m. ToC decreases12(see table 3), geologic models of thickness, thermal only from 0.16 to 0.15evolution degrees of different source rocks. and different organic-matter types are given, according to themodels established above, the gas quantities of gen026eration, adsorption, dissolving in oil, dissolving inwater and diffusion are calculated. then based on the0.22definition of the threshold value of organic enrichmentand the material balance principle, we work out thecorresponding threshold value of organic enrichmentof gas source rocks under the similar conditions, also0.l6nvestigate the major geologic factors which affect the000040005000threshold value of organic enrichmentSource rock thickness/m3. 1 The influence of carbonate gas source rocksFig. 1. The relationship between gas source rocks thickness andthickness on the threshold value of organic enrichment TOCmin(HI=500 mgHC/gC, Zam=5000 m)When the organic-matter types(hydrocarbontential Hi)and maturities(here, expressed by the3.2 The influence of organic matter types of carbon-per surface buried depth of source rocks Zam)areate gas source rocks on the threshold value of organicsimilar, analysis of the relationship of the carbonateenrichmentgas source rocks thickness and the threshold value ofOrganic matter types can be expressed by originalorganic enrichment is made. It can be revealed that the hydrocarbon potential, HI(mgHClgC), which isthreshold value of organic enrichment of gas source equivalent to original hydrogen index. Fig. 2 showsrocks organic matter decreases with the increasing gas the threshold value of organic enrichment decreasesTable 3 Fundamental geologic parameters of Tarim basin(well Tazhong 12)aStrata thickness/mFormationGeological age/Ma Paleo-geothermal gradient(C/100m) Paleo-surface temperature/C481223.32.3413K(J=0)15.1583T5.PI15.12.3415.166.5D2.6417.5570TH中国煤化工175CNMHGI75a)Lu Shuangfang, Zhao Mengjun, Peng Yan et al., Evaluation of effective stratum and potential gas source rocks in the Tarim basin (in Chi-nese), 1998Study on the threshold value of organic enrichment of carbogas source rockswith the increasing organic matter hydrocarbon poten- creasing, why does the threshold value of organic cartial. When the organic matter types are not good(hybon of carbonate gas source rocks have the decreasdrocarbon potential is low), the range of variation is ing-and-increasing law? Due to the invariable sourcelarge; in reverse, it is small. For example, the value of rocks strata thickness, gas quantities of adsorption inHI increases from 100 mgHC/gC to 200 mgHC/grocks and dissolving in water do not have great vari-then the threshold value, TOC decreases from 1. 23% ance with Zdm and with the source rocks buried depthto 0.62%, and the decreasing amplitude is close to increasing, gas quantity of generation increases at al50%0, but when the value of HI increases from 700 times, but when the value of Zam is very high(for exmgHC/gC to 900 mgHC/gC, the threshold value, TOC ample, higher than 7000 m), the increasing amplitudedecreases from 0.18% to 0.14%, then the decreasing decreases gradually; while the increasing amplitude ofamplitude is 22%. The reason of this phenomenongas quantity of diffusion is great, which results in thethat when the quantity of gas generation is definite, difference increases at first and then decreases withfrom the computational formula, we can find that the the buried depth increasing. When the value of zdrelationship between TOC and HI is approximately reaches 8500--9000 m, the gas generation capabilityinverse, as fig. 2 showsof source rocks already has been close to exhaustionbut the accumulated gas quantity of diffusion increasesconstantly with the buried depth increasing(diffusion1.2食time is extended), which results in the increase of thethreshold value of organic carbon with the increasingmaturity0.60.400.350.30100200300400500600700800900HI/mgc·gC0235Fig. 2. The relationship between the threshold value of organic en-0.20richment and original hydrocarbon potential (Hn)(Zum=5000 m, Hyy200m)0.150.10450055006500850095003.3 The influence of carbonate gas source rocksUpper face buried depth/mburied depth(maturity) on the threshold value of ar-ganic enrichmentFig. 3. The influence of upper surface buried depth on the thresholevalue of organic matter of gas source rocks(H= 500 mgHC/gC, Hfigure 3 shows the changing regulation of thethreshold value of organic enrichment of carbonate gasource rocks under different upper surface buried 3.4 Evaluation criterion of the theoretical thresholddepth(maturity) conditions. At first, the value de- value of organic enrichment of carbonate gas sourcecreases with the increasing buried depth, and thenincreases. When hI equals 500 mgHC/gC and thick-according to the definition of the theoreticalness is 200 m, the threshold value ofthreshold vanic enrichment of gas sourcedecreases to the minimum within therocks mentisfrom 7000 m to 8000 m. later it begins to increase中国煤化工 TOCmin are calculated undCN MH Gickness(100But, with the superincumbent strata thickness in- 3000 m), different upper surface buried depth(3500-9000 m), different organic matter types(type I,type II and type Ill) conditions. Then the evaluaticScience in China ser d Earth Sciencesand type Ill) conditions. Then the evaluation table of judge whether the target area is gas source rocks orthe theoretical threshold value of organic enrichment notof carbonate gas source rocks is established(see tableIn carbonate rocks sediment trap of the Tarim ba4). For the sediment trap of carbonate rocks in targetsin, most of organic matter is type Il kerogen, that iszone, if the parameters of the target strata thicknessupper surface buried depth and organic matter hydro-hydrocarbon potential is approximate to 500carbon potential are given, the fact whether the targetmgHC/gC. From table 4, we find that the correspondstrata are gas source rocks or not can be judged withing theoretical threshold value of mature gas sourcethe estimation criterion in table 4rocks in which the organic-matter type is Il, is proba-bly 0. 15%-0.2%, that is to say, in the Tarim basin, ifn conclusion, the threshold value of organic en- the content of organic carbon is lower than 0. 15%, therichment of carbonate gas source rocks is under congas generative rocks cannot turn into gas source rockstrol of the thickness of source rocks strata, organic- By calculation from the model in this paper, the obmatter types(hydrocarbon potential) and upper surface tained theoretical threshold value of organic enrichburied depth(maturity), and has variance with thement of carbonate rocks, TOCmin (see table 4 hasabove terms. Therefore, it is not appropriate to apply physical significance: under the corresponding condithe unified evaluation standard of threshold values to tions. the value is higher than ToC which is thedifferent target areasnecessary but not sufficient condition for gas generaWhen the actual geologic parameters of different tive rocks to turn into gas source rocks. And it cantarget areas are substituted into the established gas help us define exploration areas macroscopically, denyource rocks evaluation system in this paper, then thetheoretical expelling gas quantity of unit area gassource rocks can be worked out. which is used toTable 4 The corresponding theoretical threshold values of organic carbon of carbonate gas source rocksin the tarim basin under different geologic conditionspeCorresponding theoretical threshold values of organic enrichment, TOCmin(%)thickness organicat different upper surface buried depths Zam/mmatter35004000450050006000650080001.310.920.761.02420.260.220.180.160.160.150.150.20.260.160.10.161.70.860.780.750.730.710.9916II50.150.230.130.110.09141430.80.740.710.690.680161.190.290.230.180.160.150.14140.140.10.750.18140.1I0.10.090.090.111.250.990.840.750.710.690.670.70.881.070.480.250.170.150.140.140.140.140.170.160.130.l10.090.090.10.130.860.770.720690.680.670.770881.062000II0.470.170.160.150.140.140.140.150.170.11III1.090.880.790.730.7Ha0.770.881.0630000.290.0.180.160.15中国煤化工0.170210.190.140.110.090.090.13CNMHhe corresponding hydrocarbon potentials of the organic-matter types Ille…d800mgHC/gCspectivelyStudy on the threshold value of organic enrichment of carbogas source rocksome ineffective prospect areas, and reduce explora- the mature gas source rocks in which the organiction cost, in the course of directing exploration prac- matter type is Il, is about 0.25%-0.3%. In table 5, thephysical significance of the industrial threshold value3.5 Preliminary study on the evaluation criterion ofof organic enrichment of carbonate rocks, TOCgy isthe industrial threshold value of organic enrichment of under the corresponding geologic conditions, when thecarbonate gas source rocksvalue of organic enrichment is higher than ToCev, thetarget stratum can turn into effective gas source rocksIt is evident that although the organic enrichment and the following exploration stress is to observeal threshold value the gawhether there is a largdestroying formquantity of generation cannot form the industrial gas gas reservoir, well trap condition, and how about thereservoir,that is, only when the organic enrichment conservation condition of caprocks, etc, by now, "gasreaches the industrial threshold value, can gas genera- source"is not a problem. Then more importanttive rocks turn into effective gas source rocks. prospect areas in larger exploration areas can be deWhereas, at present, at least how much strength the fined by the theoretical threshold values, thereby, thegas expelling can form an industrial gas reservoir, exploration resources can be used more efficientlypaper, the lowest threshold of gas generation strength4 Conclusionsfor formation of gas reservoir, 500 m/m is used, whi-The threshold value of organic carbon of carbonch is proponed by academician Dai Jinxingith the source rocklowest threshold of gas expelling strength. And then thickness increasing, especially when thickness is less,the industrial threshold value of organic enrichment is the effect is great. The threshold value decreases withcalculated(see table 5). It is observed that in the Tarim the buried depth increasing(maturity) at first and thenbasin, the corresponding industrial threshold value of increases, from which we know that whether gasTable 5 The corresponding industrial threshold value of organic carbon of carbonate gas source rocks in the Tarim basinSource rocks Type ofCorresponding theoretical threshold values of organic enrichment, TOCy(%)thickness organicat different upper surface buried depths zin/nHyy/m matter 3500 4000600070008000850090007.6100II19.11.1.151.171.081.151.351.521.712.10.930.720.720.740.670.720.810.840.951.0633.1354.2462000.680.640.620.590.640.70.730.825.50.520.430.410.390.37040.430.458.32.21.961.541.551.541.64500II0.44040.310.330.340.370.410451.0951.481000II0.590.320.260.230.220.220.230.230.250.270.30.37140.140.150.150.170.180.990.860.860.880.910.971.131.452000II0.520.310.230.20.180.170.170.170.180.190.230.290.330.140.130.110.110.110.110.110.120.140.181.180.950.870.81II0.320.240.190.180.160.16中国煤化工131450.150.12aTHSCNMHG0.140.18158Science in china ser d earth sciencessource rocks are buried too deep or shallow( too highFrontiers, 1999, 6(Suppl. ) 195--203or low maturity) is not advantageous. 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