VICKSBURG- JACKSON SHALE. by James A. McCarthy Houston, Texas INTRODUCTION

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1 6 NOTES E 0 s VICKSBURG- JACKSON SHALE by James A. McCarthy Houston, Texas RIDGE INTRODUCTION Calhoun County, which is within the limits of the Frio formation trend, is located in the central portion of the Gulf Coast of Texas, approximately 100 miles southwest of Houston. Figure I is an area plat showing the counties surrounding Calhoun and outlining the known limits of the faulted structural fold, regional in extent, known as the Vicksburg- Jackson Shale Ridge. The structure bears the name "Vicksburg-Jackson" because the folding and faulting has uplifted these shale formations, which are unconformably overlain by the Frio formation, within a few thousand feet of the surface. Associated with and probably caused by the folding of the ridge is a large up-to-the-coast fault which strikes parallel to the main axis of the ridge in a northeast-southwest direction. On the northeast the shale ridge apparently terminates in Lavaca Bay, and on the southwest the ridge terminates in northeastern Refugio County. The shale uplift affects the beds of the lower and basal Frio approximately two miles to the northwest and southeast from its major northeast-southwest striking axis, causing a regional structure approximately twenty miles long and four miles wide. The existence of the Jackson shale formation and lower portions of the Vicksburg shale formation which have been uplifted to within a few thousand feet of the surface by folding and faulting has an important economic value to oil exploration in that several oil and gas producing fields - Port Lavaca, Sheriff, East Sheriff, Green Lake, North Sheriff, and North Green Lake, have already been found on the structure. Because of the size of the feature, it is a reasonable assumption that more producing areas will be found in the future. The shale formations, several thousand feet out of place from their normal structural and stratigraphic sequence, have been explained in different ways. A most unusual interpretation attributes the existence of the structure to "shale plugs", implying that the shale was intruded into the surrounding Frio formation. This short paper is an attempt to point out broad evidence in the existing subsurface information that demonstrates that the structure is a complexly faulted and folded buried ridge. Marginulina-Upper Frio Datum STRUCTURAL GEOLOGIC MAPS On the enclosed Marginulina-Upper Frio Datum (Figure 11) the geologic structure shows normal northeast- southwest strike and dip to the southeast,

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3 broken only by a major up-to-the-coast fault which dips 50' to 56' to the northwest. Associated with this up-to-the-coast fault is a down-to-the-coast relief fault, dipping southeast. The major up-to-the-coast fault has a throw of feet immediately above the upper Frio datum, and the minor down-to-the-coast fault which strikes parallel to the up-to-the-coast fault has a throw of feet. This minor, relief down-to-the-coast fault, terminates against the up - tothe-coast fault with depth. It is noteworthy that the deep regional anomaly -- the Vicksburg-Jackson shale ridge -- has little effect on this structural datum where the normal dip is 5O to 6O southeast which is comparable to dips found in neighboring counties which are not underlain by such a geologic feature. However, the major up-to-the-coast fault described above is a direct result of the folding and rupture of the deeper shale ridge and is responsible for the hydrocarbon accumulation between the depths of feet at Port Lavaca Field, Sheriff Field, and Green Lake Field. Oil accumulation in these fields is the result of these sands being upthrown by the fault against the Anahuac shale formation. Shale Surface Datum The datum for the map of the Shale Surface (Figure 111) is based on the first appearance of the shale, either Vicksburg or Jackson which has been recognized through paleo and/or correlation. At this structural datum the major up-to-the-coast fault throw has increased to feet and the dip of the shale surface on the downthrown block is no longer 5' southeast but 25' northwest. On the upthrown block the dip of the shale surface averages 35' southeast and at Sheriff Field approaches 50' southeast. Comparable dips on the upthrown and downthrown blocks along the length of the shale ridge are common. DIAGRAMMATIC CROSS SECTIONS On the downthrown block of the major up-to-the-coast fault there is a zone of critical dip reversal from a southeast direction to a northwest direction within the Frio formation usually below the lower Frio datum. However, isopach work reflects anomalous northwest formation dip between the interval of the upper Frio and the middle Frio, indicating that the growth of the ridge continued into late Frio time. The northwest reversal of the upper Frio and middle Frio was subsequently removed by regional tilt to the southeast. The tilt was insufficient to remove north dip from the more steeply dipping lower and basal Frio beds. On the upthrown block of the major up-to-the-coast fault the beds of the Frio formation show divergent dips, i.e., the shallow upper Frio beds are gently dipping southeast at 6O, the basal Frio beds are dipping 12' southeast and the shale surface is dipping 35' southeast.

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6 The following tabulation is a comparison of the present formation dips on the upthrown block and downthrown block of the major up-to-the-coast fault associated with the shale ridge: Upthrown Block Downthrown Block Marginulina-Upper Frio 3 SE 3 SE Middle Frio 6 SE 3 SE Lower Frio 10 SE 8 SW Basal Frio 12 SE 15 NW Shale Surface SE NW Cross Section A-A' Diagrammatic Section A-A' illustrates the north limb of the deep buried fold of the basal Frio and shale structure on the downthrown block of the up-to-the-coast fault. The section also illustrates the upper Frio formation on the upthrown block. On the downthrown block the dip is normally southeast to a point below the lower Frio datum where the dip flattens and then reverses northwest. The Brown #1 Sibley well penetrated shale (Jackson shale formation?) at a depth of approximately 8100 feet. The lower Frio and basal Frio are truncated against the shale surface, and it appears from the section that the shale was "intruded'. into the surroundding Frio formation. However, upon examination of the isopach interval from the upper Frio to the lower Frio, it is evident that the lower Frio at upper Frio time dipped northwestward as did the shale surface. The isopach of the interval from the upper Frio to the middle Frio also indicates that the middle Frio dipped northward at a lesser angle than the lower Frio. The present southeast dip of the middle and lower Frio is due to subsequent regional tilt to the southeast which removed the north reversal. Production in wells on the downthrown block of the controlling upto-the-coast fault is from the interval below the lower Frio where the original north dip is still intact. On the upthrown block of the main controlling up-to-the-coast fault the Vicksburg and/or Jackson shale is at a depth of approximately 7500 feet. Overlying the shale is the truncated basal Frio and an abnormally thin section of lower, middle and upper Frio. Superimposed upon the Section A-A', there is a gravity profile of the first derivative. The gravity values are shown for convenience on the margin of the section, and there is no correlation implied between the depths of the section and the gravity values. 18

7 The profile indicates a negative area on the downthrown block and a slightly more positive area upthrown, the whole of which is anomalous relative to a normal gravity gradient. The negative gravity values increase at the approximate crest of the shale ridge and then become more positive under the upthrown block of the fault. This increase is probably due to the shale surface which has been upthrown to approximately 7500 feet and which is more dense relative to the underlying salt which caused the uplift. Cross Section B-B' Diagrammatic Section B-B' from the North Sheriff Field area through Sheriff Field shows much the same geologic structure as Section A-A'. Once again on the downthrown block of the controlling up-to-thecoast fault, the critical dip reversal from southeast to northwest does not occur until a point below the lower Frio datum; on the upthrown block of the controlling up-to-the-coast fault, divergent dips in an abnormally thin Frio section cause truncation of the lower and basal Frio against the shale surface. Divergent dips within this interval are the reason for oil production from the Quintana #8 Boyd well which is one of the two wells producing from the deep Frio on this structure. The gravity profile of Section 13-B' shows the effect of the dense Vicksburg- Jackson shale uplifted much nearer to the surface by the main up-to-the-coast fault. There is a strong tendency for a gravity maximum along the southwestern portion of the Sheriff Field. Cross Section C-C' This Cross Section through East Sheriff Field is much the same as A-A' and B-B'; however, on the downthrown block of the up-to-the-coast fault the critical point of dip reversal occurs between the middle Frio and the lower Frio, indicating less regional tilt or sharper uplift of the Frio formation. The ridge could well be a regional uplift on which some local areas were more active than others. The gravity profile of this cross section shows the strongest gravity maximum of the three sections. Once again these maximum values occur under the upthrown block and are probably due to the effects of the dense shale being uplifted closer to the surface by the up-to-the-coast fault. 19

8 DIAGRAMMATIC SECTION A-A' sro e r FCC, NORTH GREEN LAKE FIELD AREA Ls,...,,,,. VICKSBURG-JACKSON ;{? SHALE RIDGE ff, - GRAVITY PROFILE, $ James A. McCarthy %I i ~ shdlt ~ ~ o ~ U 003 ~ ~ JX: JACKSON SYbLt

9 COI *I. 8,OOC DIAGRAMMATIC SECTION 0-8' I - L NO. SHERIFF-SHERIFF FIELDS U - UPPER FRlO VICKSBURG-JACKSON SHALE RIDGE $!F E!/i I <to,< - GR VlTY PROFILE 0 of -3 James A. McCarthv VI Vl&58L*li S H ~ ~ ~ C $000 2 ooo," Feet

10 OUINTANA OUINTANA OUlh ITANA RENWAR SPARTAN McFARLAND *I Weymeyer I)/ W~lson 111 R unk I) I Banorden I)! Nulo WI Mon~kos 4 & / -12,000' -.--.I 12,000- r 1,000' DIAGRAMMATIC SECTION C-C' :I i rcoic in fell -PER FRIO EAST SHERIFF FIELD AREA VICKSBURG - JACKSON SHALE RIDGE 5 m y!!/! James A. McCarthy ~ - GRAVITY PROFlLE V X : VICKSBURS SHALE 1,~00 IX! JICKSON SHALE

11 SUMMARY AND CONCLUSIONS From the foregoing discussion, the subsurface evidence points strongly toward a natural sequence of geologic events that resulted in a complexly folded and faulted regional structure rather than a series of local "shale plugs" that intrude into the surrounding Frio formation, having little or no effect on that formation. The general dip of the upper Frio increasing with depth to the shale face where the dip increases to a very steep angle is indicative of an uncomformable surface where uplift continued into Frio time, but the amount of uplift was not comparable to what the older formations (Vicksburg and Jackson) underwent before basal Frio time. The formation dips of the structure before regional tilt of the Frio indicate structural growth gradually dying at middle and upper Frio time. On both the upthrown and downthrown fault blocks of the up-to-the-coast fault there are portions of the basal Frio absent and much of the Vicksburg formation absent, indicating erosion. Figure IV is a series of individual cross sections used here to present this writer's interpretation of the sequence of geological events which caused the growth of this structure. The first cross section of Figure IV shows the Jackson-Vicksburg shale being deposited in &I essentially horizontal position. The second cross section of Figure IV shows the Jackson and Vicksburg shale formations undergoing sharp folding which was probably due to salt intrusion at depth. The third cross section indicates that the uplift and folding became excessively intense causing rupture and faulting (up-to-the-coast). In cross section 4 continued uplift during late Vicksburg time and early Frio time caused emergence and subaerial erosion of the strwture. The structure was exposed to erosion for a long period of geologic time, resulting in exposure of the Jackson shale formation. Actually there was probably more than one period of erosion which was caused by differential movement along the continually growing up- to-the- coast fault. Cross section 5 of Figure IV shows continued uplift, erosion and displacement along the fault with a portion of the basal Frio section removed by erosion. Cross section 6 depicts the end of the uplift, regional tilt to the southeast (thereby removing the structure reflected in the middle and upper Frio beds) and continued displacement along the up-to-the-coast iault, accompanied by upper Frio deposition.

12 SUGGESTED STRUCTURAL GROWTH ALONG VICKSBURG-JACKSON SHALE RIDGE, CALHOUN COUNTY, TEXAS 6,000 - I Depos~l~on of Jocksan 8 V~cksburg sholes 2 Foldtnq of Jacksan 8 V~cksburg Sholes 3 Up to-the-coast faulting. 4 Erosion of V~cksb~rq ond 8nlhoI deposlhon of Bosol Frlo 5 Renewed upl~ft, eroslon and dlsplacemenl on fault Eros~on of o porllon of Basal Frla 6 End of upl#ft, continued d8splocement an fault Reglono1 tilt sa~'h and coverlng of Ridge by Laver Fria, Mlddle Frlo, and Umer Frla sands and shales FIGURE IT VICKSBURG-JACKSON SHALE RIDGE '?%tn FR /#P James A McCarthy i OWFR FRIO 8 81SP. FRO U MIDDLE R 0 \i i I.i,I L I N - A t