..J cPTb{,-03-- Ob Quantitative log Evaluation of the Prairie Evaporite Formation In Saskatchewan By E. R. CRAIN and TV. B. ANDERSON' (17th A.nnual Tecllnical ~lIeeting, The PetTole1.t'm Society of C.l.lll" Ed'manton, llfaYJ 1966) Downloaded from http://onepetro.org/JCPT/article-pdf/5/03/145/2165444/petsoc-66-03-06.pdf by guest on 27 September 2021 ABSTRACT 'I'he problem of solving for the fraction of s:rlvite7 car~ nallite, halite and insoluble material in the Prairie Eva­ porite formation can be performed. by a suitable inter­ pretation program based on Gamma Ray, Sonic. Neutron and Caliper logs. Empirical relations were established between the log '"aloes and the formation parameters, the result being- a set of four simultaneous equations which may be reduced to obtain the desired fractions_ Tedious hand calculation can be eliminated by using computer techniques and automatic log digitizing machines. Correla~ tion between core and log analysis is good, and the speed and efficiency of the method is valuable in initial forma­ ,_:.: .. tion studies. INTRODUCTION HE Prairie Evaporite formation has been the object of extensive study in the past several years , T(1) (2). It is the richest known potash-bearing bed Figw"e 1. .-~' I in the world, and, as such, it is important that any .. information gathered concerning- the zone be accurate POTASH CONTENT and immediately useful. NOMOGR,aPH Electrical and radioactiVity well logs have proved ,~, to be of value for formation evaluation in the oil in­ dustry. A recent paper (3) illustrates their use for both qualitative and quantitative interpretation in .. ~ evaporite sequences in various parts of North Amer­ ica_ .i I This paper will outline the theory and technique .~ .' '~ used for a quantitative interpretation procedure in -J I the potash beds of the Prairie Evaporite formation in '-.: . <:,"-,- the Province of Saskatchewan. The data are set. up , "",..,-0<1[<_ -.: '- ........ "'< u.>'T~I"'l :: ..--- so that the}r can be handled by an electronic computer. ~,TIt . llRlUlO:rrUT_' .....r•• ,,'to _'0.u_n"". 11 The equations can also be computed by hand at the .lO'l'II'an ...... 'JT ~.' f well site to supplement the data already available_ The computer program is presented as an Appen- I· dix. An extensive bibliography! covering potash geol­ ., oglr, development and logging techniques, is included, Figure 2_ ,. THEORY Figltre 2 illustrates a nomogram which facilitates It is well known that potassium has a radioactive conversion of gamma ray activity to apparent K::!O k;otope 'which emits gamma ray energy. This isotope content. It is derived from the graph of Figure 1 (K4U) comprises a constant fraction of the total amount and therefore gives the same results. The result is labelled "apparent" K~O content because the insoluble of potassium, 50 that a Gamma Ra,}r Log, which meas­ r ures the amount of natural radioactivity in a forma­ content of the formation generally is slighth radio­ tion, frequentllc gives a measure of the potassium active and the chart tllus gives an incorrect K::!O value if insolubles are present. A correction can be applied, I content. which will be dealt \\·ith later, but it is small in many Considerable \II,.-ork \vas done in 1964 to establish cases and will not greatly affect the total ~O value of an empirical correlation between gamma ray activity a zone. These results are valid only for the logging and the K,O content of a potash bed (3). The graph tools listed on the chad and bor beds thicker than shown in Figure 1 illustrates the results obtained in 2 feet. oil-base muds. As borehole conditions affect the re­ sponse of gamma ray logging instruments, hole size and mud weight must be taken into account. :;·Schlum,be1'um' of Canada, CalgaT1.I, Alta. Technolo9V, July-September, 1966, Montreal For thin beds (from 1j~ foot to 3 feel), a con-eetioH can be made using the empirical chart shown in Figure :]. The gamma ray reading in API units i~ rnultiplietl by the conection foetor derived from Figure 3 to ar­ l-iYe at the corrected valuE'_ Fnr beds less than 1 foot in thickness, the correction be('omes quite large and o ~ is not accurate. Bed boundaties are chosen at the in­ flection points of the gamma ray curve and the b~d , thit:kness is that distance between any two successive inflection points. -:++-H~4.7V: o As onl.y thfJ,Se zone:.; which are low in carnallite content aloe commercially athactive at pres~nt, a means of delineating these beds must be employed. The N eu­ i~:~~~~~:~-!:: ~'~B ! '::I,l~ ?:il~~tl-,,~: f1~f:~f~;~:: tron Log is an excellent carnallile logging tool, because " it responds to the hydrogen content of the formation. ,-,- '+Ji :HII:n',i,'~L."'~;fi::,abLfr"',"" "~::,~'-:',:.'~',,, The water of hydration associated with carnallite comprises a large part of its volume, so that a zone Figure .1. rich in carnallite will have II large hydrogen index. 65 The hydrogen index of pure carnallite is per centDownloaded from http://onepetro.org/JCPT/article-pdf/5/03/145/2165444/petsoc-66-03-06.pdf by guest on 27 September 2021 (4). SylVite and halite have an index equal to zero, except for a small (1 to 2 per cent) volume of included water. Again, the insoluble content of the zone affect~ this log, and it should be taken into account if it is found to be very large (greater than 5 per cent 1. An empirical chart similar to Fig/ll"i' .4 can Le made for each well to be interpreted tf) obtain the h~rctrogell index_ A pivot point at GOO API units and a 65 pel' cent hydl·ogen index defines olle end of the straight line lOll tiemi log paper 1 _ The Neutron Log value (API units) in a clean salt :~one and a 1 per cent hydrogen index define!; the other end. The pivot point used for this example applies only for the tool, spacing and source type noted on the chal·t. Different pivot points must be determined for different tools. Precise interpretation using the Neu­ tron Log is limited to beds thicker than 2 feet. A Sonic Log is employed as an aid to determine the insoluble content. This is a required fador if an accurate interpretation is to he made. Knowledge of the insoluble cuntent is al!io necessary bQcause exces­ sive amounts of insolubles can make an apparently good zone commercially unattractive, as it is an ex­ pensive process to refine these imllUritles from the final product. Studies on labol·atory samples and field con-elatiom:\ (3) have given ~;onic travel time \"ulueH in halite, sylvite, carnallite and insoluble material as 67, 74, 78 and 120 microseconds per foot respe.diveb'. Thifl data. combined with the inf(lrm~ltion which can be NEUTRON DEfLEc:nON - API IHIITS determined from the Gamma Ray Log and the Neu­ tron Log, can be used to set up simultaneouR equa­ Figure 4- tions to solve for the per cent of halite, sylvite, car­ nallite and insolubles (w, x, y and 7. respectivelY*l (51. Table I shows the values used for the coefficients of the equations. These coefficients represent values TABLE I of the parameters for 100 per cent pure minerals. COEFFICIENTS FOR POTASH LOG EVALU,~TION A~ it is a:isumed that only halite, .sylVite, carnallite and :lome insoluble:i are present, their total volume mllst comprise the total formation volume. This is IHillCTal Halite Sylvite Carnallite Insolubles represented by equation l. w y z Symbol w + x + }! + z = 100 equation AppaTeIIl /(20 Conlent 0,00 0,63 0.17 The total apparent K~O content of the formation, (fractional) based all the Gamma Ray Log. is made lip to each com­ re~peetive K~O Hvdrogw Index 0.00 0,00 O,G5 0.30 ponent fraction multiplied by its value_ - (fractional) Thus: 0_63 :.. 0.17 Y 0.05 z = K:!O""" cqll~tiun 2 Sonic Travel Time 67.0 74.0 78.0 120,0 + + (microseconds per [oot) -!<Symbols defined in Appendix. 1. 14~ The Journol of Canadian Petroleum --~ --~-_ .._- -' --' ','-- TABLE II EVAPORITE MINERAL PARA]\,IETERS Apparel'lL Sonic K,O Hydrogen Apparent True Travel Rating Index Density Density Time 1\1illerat Grams per Grams per cubic cubic lVIicroseconds Per cent Per cent centimeter centimeter per foot { Anhydrite.. .... " .... 0.0 0.0 2.98 2.96 50 Carnallite... ... -.. .. .. , . ... 17.0 65.0 1.57 1.61 78 Gypsum .. _. .... .. - .. .. .. - 0.0 49.0 2.35 2.32 52 Halite ... ..... .... -- ....... 0.0 0.0 2.03 2.16 67 Kainite...... _. .... .. ' .. _.- 18.9 45.0 2,12 2.13 - Langbeinite... ... 22.6 0.0 2.82 2.83 52 Polyhalite. _. ... ...... - . 15.5 15.0 2.79 2.76 57 Downloaded from http://onepetro.org/JCPT/article-pdf/5/03/145/2165444/petsoc-66-03-06.pdf by guest on 27 September 2021 Sylvite. __ .. .. ... .. .. ....... 63.0 0.0 1.86 1.98 74 Insoll1bles. _. .. ... - ... .. ' ... .., . 5.0 30.0 2.60 2.60 120 ;-".. ., ~ .. .!.' ;'-'.' "016.0 't. 3~DO. lO.S ~ b.l ~.) oJ />.1 89.3 4o.'Ii .1 ... ~ ~DI5.5 ez. J~5D. TO., . 8.1 4.oIr .s '.9 90.1 3_5 D.D ].1 _,. -';01".00. _ 60•• 3l6D•.__ 'O.~ __.6.1, _J.~ __l.:\ -----l._.__BJ.:4 __ J._D ...__...._~ • __ 'liOt9.5 1.r;0. ~lO(l. 'o.~ 6.l Z_g 3.2. 13.~ BO.~ 9~3 .5 B.1 - ~020.0 u,,~ 3100. '2~O 6.l ~.~ 2.0 1~.1 n.) 1I~7 .1 ll.~ _~o2e.~ 22'.