- 169 - Lithofacies of the Lower Cretaceou s Mannville Group , We st-Central Sa skatchewan by J . A. Lorsong A de t ail e d sedimentol ogical i nvestigation of the Mannvi l l e Group i n the Lloydminster a rea was ini t iat ed in earl y 1979 . The aim of the study is the development of a facies model which will aid in explor ati on f or and development of heavy oil-beari ng sandstone bodies. Preliminary descriptions of t he cons tit uent lithofacies of the Mannville Group a re presented here . Descr iptions a re based on det ailed study of 500 m of core from t he upper and middle parts of the s uccession (Colony , McLaren, Was e ca, Sparky , General Petroleum and Rex formations) in t he norther n por tion of the Lloydminster a r ea. Preliminary exami nation of cores f rom other f ormations and other areas s uggests that t he descr iptive sche me outlined below is applicable to the Mannville Group thr oughout west-centra l Saska t chewan. The Mannville Group compris e s a poorly consolidated sands tone-shale s equence approxima tel y 200 rn thick. The s uccessi on has been divided into nine f ormations for the purposes of geophysical l og interpretat ion and pr oduction accounting (Vigrass , 1977 ; White & Osinski, 1977). Oil is produced from numerous s andstone bodies which typicall y range f r om l m to 6 min t h ickness. A singl e vertical section of t he Mannvi lle Group may i nclude 30 or more s uch sands t one bodies . All sands tones ar e lateral ly disconti nuous; many pinch out over a di stance of 250 m or less. The limited a r eal ext ent of i ndividual sandstone bodies thus precludes accura t e delinea tion of r eservoir geometry by drilling, even at the minimum (f i ve- acre) well s pacing . De t ailed information i s r equire d, however, f or modelling of fluid b ehaviour and efficient reservoir deve lopment i n enhanced recovery project s . It is necessary theref ore, to deve lop a facies model with which probable reser voir geometries can be predicted on sedimentological criteri a. Description of Li t hofacies Facies L (low-angle cross-laminated s andstone ). Faci es L consists of ver y fine to fine- grained, well sorted quartz sands tone with very minor intercalations of shal e. The faci es is cha r acterised by low-angle (generally l ess t han 10°) multi- directional cross-lamination. The l amina e as s een in cor es appear to be planar. Individua l sets vary from 5 cm to 250 cm in thi ckness (mean 34 cm) . Low-angle cr oss­ lamination a c counts for about 60 pe r cent of facies L; t he r emainde r comprises lar ge scale (sets ~10 cm) and s mall scale (sets <10 cm) trough cross-lamination , - 170 - and high angle, apparently planar, cross-lamination in approximately equal pro­ portions (Fig. lA). Trough cross -lamination is t ypically most fully developed near the base of each facies L i n terva l and gradually diminishes in i mportance upward. Nearly all of the petroleum production in the Lloydrninster area is from facies L s andstone bodies. Facies T (trough cross-lami nated sandstone ). Facies T comprises very fine to fine­ grained, mode rately sorted quartz sandstone, generally with small proportions of silt and clay. The facies is charact eri sed by multidirectional trough cross­ lamination, low-angle cross-lamination, and horizontal laminati on (Fig. lB). Sets of trough cross-lamination are up to 100 cm and average about 20 cm thick. Facies M (massive sandstone) . Facies M s andstones are generally fine to medium­ grained, mode rately t o poorly sorted, and contain substantial quantities of silt and c l ay. Sedimentary structures are virt•1a lly absent, although remnant traces of cross-lamination suggest that some facies M i ntervals may represent intensely bioturbated equivalents of facies Tor L. Facies B (bioturbated s ands tone and sha l e). Facies B comprises very fine to fine­ grained, well sorted sandstones and interbedded shales. Bed thicknesses range from 0.5 cm to 25 cm (mean about 5 cm). The proportion of sandstone varies from 10 to 90 per cen t and averar,es approxill!ately 40 percent. Near ly all sandstone beds exh ibit small scale trough cross-lamination, either in multiple sets or in isolated bedforms. Preserved bedforms include symmetrical oscillati on ripples (more common) and asymmetrical ripples (less common). Orient­ ations of cross-laminae and of ripple crests vary considerably between adjacent beds. Mass ive and parallel-laminated sha l e beds are about e qually common. Shale layers commonly conform to the shapes of subjacent bedforms to form intervals of lenticul ar , wavy and f l aser bedding (in order of decreasing abundance; see Reineck & Wunderlich, 1968 for terminology). Upper parts of shale l ayers are commonly cut by sand-filled desiccation cracks . Facies Bis characterised by pervasive bioturbation. Intensity of bioturbation ranges from isolated burrows (both horizontal and vertical; filled with either sandstone or shale) to i ntervals of thoroughly intermixed sandstone and shale in which primary depositional structures cannot b e distinguished. Facies S (shale). Facies S includes massive intervals of grey shale (subfacies - 171 - S1) and simila r intervals marked by nume rous lami nae of white or l ight gr ey s ilt (subf acies s2). Subfacies s2 dis play s many f eatures common in facie s B, but on a s malle r scale. Silt layers are t ypi cally 0 . 1 cm to 0.5 cm thick, and generally exhibit isola ted r i pples (mor e commonly) or horizontal lamin­ ati on (le s s commonly). Graded laminae a r e obser ved occasion- a lly. Lent icula r, wavy and f laser bedding a re commonly developed, as are desiccation cracks filled wi th silt. Facies C (coal). Facies C compri ses thin beds of black vitreous lignite and carbon­ aceous black s hales which commonly overlie lignite layers . Organic ma t e rial consists primarily ofte r­ r e strial plant debris . Coal Fi g. 1 - Typi cal appearance of predominantly l ow-angle beds a re typically a ssociated cross-l ami nation in f aci es L (sec tion A) and trough cross-l amination in fac ies T (Section B). with l ea ched zones (in sand­ Heavy l ines i ndi cate erosional surfa ces ; l i ghter lines show incl inations an d sha pes of laminae . s tone ) into which root tra ces extend 50 cm or farther. Distribution of Lithofacies Relative proportions of lithof acie s in the upper part of the Mannville Group are s hown in Table 1 . Faci es L, B and Sare the most abunda nt; faci es Tis ubiqui tous , but less important volumetri cally ; f acies C int erva ls are mode rately common, but are s ubs tantially thinner t han other fac ies; facies Mis not commonly developed. Although s everal f acies share conunon charac t eris tics, each is discre te in - 172 - verti cal section and is easil y distinguished from adjacent i ntervals . Facies L, T and Mare composed almost ent irely of s andstone; t he f aci es differ somewhat i n grain size and sorting, but the primary dif ferences a re i n sedimentary s tructures . This suggests that the faci es may be spatia l l y associated. Howeve r, such a rel ation­ shi p i s not observed in vertical sequence: nearl y ever y s andstone body corresponds uniquely to a single facies . Lateral a ssociations of facies a re more di fficult to assess . Tentative correl ation of closely spaced cores s uggests t hat facies L sandstone bodie s may gr a de into faci es T over dis tances of 100 m or less in some instances. Other facies are apparently independent over similar distances . Table 1 facies vol ume tric numerical t hickness (cm) proportion prorortion* maximum mini mum mean L (low angle cr oss- lamin- 37% 24% 744 26 231 ated sandstone) T (trough cross- l aminated 13% 13% 431 24 143 sandstone) M (massive sandstone ) 3% 3% 252 16 142 B (bioturba t ed sandstone 28% 27% 486 20 158 and s ha l e) S1 (massive shale) 8% 13% 697 17 90 S2 (laminated shale) 10% 14% 459 20 l i)3 c (coal) 1% 6% 154 6 28 * pr oportion of 286 measured intervals Discussion Six distinct lithofacies ar e represented i n the Mannville Group in the Ll oydminster area . Although a thorough interpretation will depend upon f urther study, t he sali ent cha racteristics of the facies suggest some prel iminary inter ­ pretations. Low angl e cross- lamination in faci es Lis typical of swash and hummocky stratificati on associated with beaches and shallow offshore enviromnents (Harms & others , 1975 ; Hamblin & othe rs, 1979). Evidence for migration of sand waves - 173 - (high-angle c ross- lamination) and dunes and ripples (trough cross-lamination) is present in both facies Land T. Wave action and possibly unidirectional currents are indicated by oscillation and asymmetr ical r ipples in facies Band s 2. Lamination in subfacies s2 closely resembles the pin- stripe bedding characteristic of tida l flats (Reif & Slatt, 1979; Thompson, 1968).