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By LK Kreis the Study of the Oil-Prone Jurassic Section in the Wapella

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By LK Kreis the Study of the Oil-Prone Jurassic Section in the Wapella - 176 - Regional Stratigraphic Correlation and Lithology of the Jurassic, Wapella - Moosomin Area, Southeastern Saskatchewan by LK Kreis Kreis, L.K. (1987): Regional stratigraphic correlation and lithology of the Jurassic, Wapella - Moosomin area, southeastern Saskatchewan; in Sunmary of Investigations 1987, Saskatchewan Geological Survey; Saskatchewan Energy and Hines, Miscellaneous Report"a7-4. The study of the oil-prone Jurassic section in the this contact. Vigrass (1952) records the presence of Wapella - Moosomin area was initiated in June of a zone of bluish chalcedony at the contact and the 1986. The focus of the study is to develop a variable thickness of the Lower Gravelbourg. depositional model by a detailed study of all available core, geophysical well logs and, where Christopher (1984, p.90) observes that a sandy necessary, drill cuttings. facies overlying a hidden unconformity exists in the upper half of the Upper Gravelbourg Member as far This paper presents a regional stratigraphic west as longitude 105ow. He suggests that channels correlation for the Jurassic between type and study have truncated the underlying units of the Upper areas (Fig. l}, and introduces the dominant Gravelbourg and, as one approaches the arch lithologies encountered in core taken from the (unamed in the paper) along longitude 102ow, even undifferentiated Middle Jurassic (i.e., Shaunavon - the Lower Gravelbourg Member has been removed. Upper Gravelbourg equivalent) section in the Red This upper sandy facies then disconformably Jacket - Moosomin localities (see section in overlies the Upper Watrous. accompanying map package). Work done to date suggests that there is no recognizable Lower Gravelbourg section present in Stratigraphy the study area. The evidence cited by earlier workers for the presence of a disconformity (bluish The west-east cross-section (in accompanying map chalcedony, variation in Lower Gravelbourg package) shows the correlation of Jurassic strata thickness areally) has also been observed to the between the type area in southwestern southwest of the study area. The author believes Saskatchewan (as defined by Milner and Thomas this to show that the.Lower Gravelbourg thins (1954)) and the presently active petroleum northward due to a combination of depositional exploration area of the southeast. In particular, it onlap and an apparent post-Lower Gravelbourg demonstrates the correlation of the type section for erosional event in this the northeastern margin of the Shaunavon and Gravelbourg Formations at the Williston Basin. The top of the Lower Eastend Crown No. l (15-l l-6-20W3) with the Gravelbourg appears to be disconformable for a equivalent section in the study area. Note the rapid considerable distance beyond the study area. and pronounced facies changes over this interval east of well 10 on the cross-section, leading to an Correlation with strata in the type area has also arbitrary cutoff being made for Shaunavon and permitted differentiation in the study area between Upper Gravelbourg correlation between wells 10 the Rierdon Formation of the Vanguard Group and and 11 . However, it appears that correlation of the Shaunavon-equi valent strata. In the type area, Lower Gravelbourg dolomitic limestone strata may there is sufficient variance to make a pick between be possible for some distance further east. the rather uniform resistivity and other log signatures for the calcareous shale of the The Lower Watrous Member of the Watrous lowermost Rierdon beds and the generally sandy Formation pinches out against the Mississippian limestones of the uppermost Shaunavon beds. unconformity between wells 13 and 14 in the east. However, travelling east of the arbitrary cutoff The Lower Watrous is overlapped by Upper Watrous (between wells 10 and 11) on the cross-section, in well 14. there is an increasing amount of clay in the uppermost Shaunavon equivalent sandstone beds. The Lower Gravelbourg Member also exhibits This results in much less variance between log pronounced thinning and eventual pinchout against signatures of the Rierdon and uppermost Upper Watrous strata between wells 12 and 13 in Shaunavon-equivalent beds, and the pick becomes the eastern portion of the cross-section. ln more tentative. wells 13 and 14, the Lower Gravelbourg appears to be absent and is overlapped by strata equivalent to Christopher (1964) presents an exhaustive discussion the Shaunavon and Upper Gravelbourg. of the Rierdon/Shaunavon contact in southwestern Saskatchewan, and places the contact at the base of Earlier workers such as Vigrass (1952), Stott (1955) a thin basal sandstone unit with abundant well-worn and Christopher (1984) suggest that the top of the Gryphae nebrascensis and other pelecypod shell lower Gravelbourg is disconformable. Stott (l 955} material. This basal Rierdon sandstone unit appears cites as evidence distinctive changes in lithology at to be quite similar in lithology and log character to - 177 - T. RG. I RG.. 33 FIG. 32 ~G. 31 FIG. 30 j, l ··~·-,r .... L_ l '" '" -~ J I ·j I ~ i ' I .-+--~~ I -i ' I , : ' ------r­ ---4 I j ! ··· '• t, .· . 1~ i' . ! -~· ;1 . r---;. t· ·---t---. I j. ' - ________. ·-- - i I ct- ·--~ : :.-.. ; r· -: --· • ..: ... '.- .1 I ,.;:., ·-- --.-- '!. ,~·- ., ""; _ _.l ··-----,.- ·-t .j---+ ' ! ·-r j ·t 1_ ~ - ··1· -+i ·1~ ~ .... 1- _. I -~ ~ ~ j ~-4: r ·- + -· L ' Ki1 om~1,u Figure l - Study area showing main petroleum producing localities and position of cross section A-A' in Fig. 2. an apparent basal Rierdon sandstone unit present in Similarly, in the present study area, it may be some wells in the present study area. In effect, the impossible to determine whether the argillaceous, base of the Rierdon (i.e., lowermost Vanguard) is fossiliferous (G. nebrascensis-rich), calcite­ placed by Christopher ( 1964) at the first appearance cemented sandstone belongs as a time-rock unit to upwards of G. nebrascensis. In a later publication, the Rierdon. As in the southwest, it may represent Christopher (1966) revises some upper Shaunavon an early invasion of G. nebrascensis during Upper picks and suggests that there may have been an Shaunavon time. early invasion of G. nebrascensis from the north leading to the presence of G. nebrascensis in his Well 2 on Fig. 2 illustrates the log response of this uppermost U3 unit of the Upper Shaunavon in some well-worn G. nebrascensis-bearing sandstone unit wells in the northern portion of his study area. and the contact between the Rierdon and the Upper - 178 - PE RIOD M,6,NNYII..LE CIHTACEOUS RIERDON ~---+----.----,+.,.'-,,---- -----­ ,G. nebrocens15 sonds.tane SHAUN A.1/0N- UP?fA GRAYELBOURG EQ UIVALENT JURASSIC ,,.-·· UPPER WATFWLJS MISSISSI PP IAN MISSISSIPPIAN > Pt I ~OL rLJM ~RODUCH<G LONES. Figure 2 - Cross-section A'A' showing cored intervals exhibiting the dominant lithologies encountered in core in the study area and described in Appendix A. Also shown are zones of present petroleum production in the Red Jacket and Moosomin localities. Shaunavon equivalent. The top of this sandstone Occasionally, thin, very poorly developed, poorly unit defines the top of the Shaunavon - Upper sorted, highly argillaceous sandstones occur near Gravelbourg equivalent interval in this well. A the base of the Rierdon. They rarely have enough detailed description from core, including the lithologic variability to be seen on a geophysical log apparent contact interval, may be found in and are thus unlikely to be included with the Appendix A. generally sandier underlying Upper Shaunavon lithologies. Since it is not possible to distinguish on geophysical logs the known G. nebrascensis-bearing sandstone The cored interval between 653.1 and 653. 3 m (and from other sandstone which may be Upper likely most of the missing core between 653.3 and Shaunavon-equivalent, the author chooses to 655.0 m) of well 12-4-l4-32Wl (Appendix A), include the G. nebrascensis-bearing sandstone in provides an example of such a sandstone which the Upper Shaunavon-equivalent interval, realizing cannot be distinguished from the calcareous shale that this sandstone unit may belong to the Rierdon. of the Rierdon on the geophysical log (Fig. 2, well 2). In addition, this sandstone overlies the G. Where no sandstones are developed, calcareous nebrascensis-rich sandstone discussed above andis shales of the Rierdon rest upon sandy shales, shaly separated from it by a ID cm (655.1 to 655.2 m) sandstones or interlaminated shales and argillaceous calcareous mudstone with abundant G. nebrascensis sandstone beds of the Upper Shaunavon equivalent, and one 3.5 cm long almost complete belemnite all of which are calcareous. This results in little guard. Belemnites are common in the Rierdon and contrast in geophysical logs between the Rierdon have never been reported in the Upper Shaunavon. and the underlying beds. In these instances, careful This would place the 10 cm thick calcareous geophysical log correlation with wells that have mudstone and the overlying sandstone within the contrasting Upper Shaunavon-equivalent Rierdon. sandstones, or correlation to the cored section is desirable. Picks made away from the control wells must be considered as tentative. - 179 - Lithologic Descriptions appears to represent a transitional unit underlying the oolitic limestone. Although a continuous core through the entire section does not exist in the study area, a general This argillaceous sandstone unit is underlain by a representation of lithologies encountered can be 1.0 m thick conglomerate consisting of subangular made by selection of various cores from different to rounded clasts of dolostone, lesser chert and stratigraphic intervals. The cored intervals used minor scattered quartz. In the opinion of the are indicated on the cross-section in Fig. 2 and are author, this conglomeratic unit represents a described in Appendix A. This cross-section also regolith of uppermost Upper Watrous strata and can illustrates the zones of current petroleum be cited as evidence for exposure at this surface. production in the Red Jacket - Moosomin locality. The author believes the disconformity here to be contiguous with the previously discussed The cored intervals in well 2 (Fig.
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