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'rQ: mwd sw..j"t: f CiU:i ~Û ,¡;n: t'b~ to ~it* It 2 ec: Ot'ig. & 2 Oil £ Chi~£) Bf~C- 1 , CONFI DENTIAL . e APRIL 1969 GEOLOGIC REPORT ACCO¡ItPANYING APPLICATION FOR DESIGNATION OF PROPOSED TAvJAWE LAKE UNIT INTRODUCTION The purpose of this report is to present geological evidence in support of the Application for Designation of the Tawawe Lake Unit. Attached hereto, as an integral part of this report, are the following exhibits. Exhibit A. Structure Contour Map, based on seismic data, with outline of proposed unit area and with structural X-section from the proposed location to Union Oil Company1s Tazlina Unit #1. Exhibit ß. Regional Geologic Map, showing geologic and geographic setting. Exhibit C. Composite Log - Union Oil Company1s Tazlina Unit #1. The proposed Tawawe Lake Unit is located in the south-western part of the Copper River Basin, approximately 150 miles northeast of Anchorage and about the san~ distance north of Valdez by way of the Richardson Highway. Access to the proposed initial location can be obtained by building seven miles of roaa north from the Gl enn Hi ghvJay in the vi ci ni ty of ~1ende ltna Lodge, STRATIGRÞ,PHY , The Copper Ri ver Basin is a mari ne ~!¡esozoi c provi nee defi ned by outcroþs on the western and eastern flanks and by, Mesozoic rocks encountered in seven widely spaced wildcat wells. Most of the basin, including the proposed unit area, is covered by Pleistocene and Recent glacial and alluvial deposits, For petroleum exp 1 orati on purposes, Ibase:,1ent" rocks in the proposed unit area are hi 9h ly altered volcani cs and sedi ments of the Early Jurassi c Talkeetna Fonnati on. These rocks are well exposed on the western and southern edges of the ~asin. Overlying the Talkeetna Formation is a sequence of Middle to Late Jurassic shðle, siltstones and sandstones very similar to rocks found on the Iniskin Peninsula. These rocks , have been sub-divided into a basal, coarse clastic unit designated as the Tuxedni Group, a ddle unit comprised primarily of fine clastics called the Chinitna Formation and an upper unit consisting of fine and coarse clastics called the Naknèk Formation, The Tuxedni Group, well exposed on the western rim of the basin, includes 2400 feet of sands tone and conglomerate whi ch have never been tes ted in a wi ldcat Vie 11. The :~e 1 chi na Formati on of Early Cretaceous age unconformably overl i es tne Nak;'1ei< Formation and consists of 800-1200 feet of limestone and sandstone with good reservoir Do~ential. Rocks similar to those seen in the outcrop have not been encountered in any Copper River ßasin wildcat probably because of complex faulting and stratigraphy, The Late Cretaceous ~'1atanuska Group ove:~lies cche Nelchina Formation. The t~atðnuska Group is approximately 14,000 feet thick and is composed primarily of dark gray marine lutites v:ith coarse clastic units in the basal and uppermost portion? These rocks are well exposed in the Nelchina are~, and mos~ of the Copper River Basin wilacat " If'- CONFI DENTI.~L . e Page 2. wells have penetrated fairly thick sect~ons of Matanuska shales and siltstones. Tertiary non-marine sediments are sparsely exposed in this area, and the maximum thickness in the basin does not appear to be over 200U feet. The thin veneer of Pleistocene and Recent alluvial or glacial deposits seldom attains a thickness in excess of 500 feet, STRUCTURE The structure shown on Exhibit A has been maQJ2d exclusively by seismic methods, and the contoured hori zon is a sei smi c IIphantO:n" near the base ot" the Lovler Cretaceous section. The unit outline encompasses three north-east trending structures separated by faults which appear to be part of the Castle Mou~tain Fault system. The structure west of fault IIßII is a doubly plunging anticline ~\!ith 16uO feet of vertical closure above Uni on IS #1 Tazl i na we 11. The structure east of Faul t II ßII is essenti ally a north\>Jes t dipping homocline bounded on the south-east by Fault IICII. ¡he structurally high level of the phantom horizon on the northvv2st side of Fault IICII in addition to indications of dip reversal and contour closure just northeast of Tazlina Lodge are the reasons for including this structure in the unit area. The structure east of Fault IIC" appears to be geologically analogous to the adjoining structure to the west, but seismic control at the southwest end of this structure is not available. Several angular unconformities within the Mesozoic section have been mapped on the surface; therefore it is difficult to predict accurately the distribution and thickness of rock units in the subsurface. An attempt to identify the seismic phantom hori"zon was made al Union Oil Company's #1 Tazlina well. It appears that the phantom is near the base of the Cretaceous or just above the Cretaceous-Upper Jurassic unconformity, but the identification is somewhat tenuous because a seismic line through the Union well is not available. It is expected that the rock units in the initial test well will closely resemble the section encountered in the Tazlina Unit well, but there is a good poss i bil i ty of encounteri ng a sedi mentary seed on with better reservoi r potential because the test well should penetrate Jurassic rocks not reached by the Tazlina well, and also because the test well is 5 miles closer to good reservoir rocks in the Nelchina outcrop area, PROPOSAL TO DRILL The tentative location for the initial test well in the proposed unit is within the SW 1/4 of Section 24, Township 4 North, Range 8 West, C.R.M.. The well will be drill ed to corrmerci a 1 producti on of on or gas or to a depth 300 feet into the Lower Jurassic Talkeetna Formation, unlèss at any time the unit operator shall establish to the satisfaction of the Supervisor that further drilling of said well would be unwarranted or impractical; however the operator shall not be required to drill said well below a depth of 6700 feet, ' Mob il Oil by: