E. Texas Upper Jurassic reefs: An expanding Cotton Valley lime play

Sept. 22, 1997
Exploration for gas reserves in reefs of the Upper Jurassic Cotton Valley Lime (Haynesville) interval, East Texas basin, provides an excellent example of a frontier play in a maturely drilled province.
Scott L. Montgomery
Petroleum Consultant
Seattle

Gus Wilson
Saker Geological Services
Houston

Dan Ziegler
SK Resources
Houston

Keith Hatch
Sigmund, Kane & Hatch Inc.
Houston

Exploration for gas reserves in reefs of the Upper Jurassic Cotton Valley Lime (Haynesville) interval, East Texas basin, provides an excellent example of a frontier play in a maturely drilled province.

Small, pinnacle-shaped reef bodies, 15-100 acres in area and 250-650 ft or more in height, were not known to exist in East Texas prior to their serendipitous discovery in the early 1980s despite the existence of over 900 wells penetrating the Cotton Valley Lime interval. To date, reefs have been found productive along a fairway 6-10 miles wide and up to 75 miles long trending northeast through Robertson, Leon, and Freestone counties (Fig. 1 [41,679 bytes]).

Both 2D and 3D seismic data have identified reef-type anomalies over a total prospective area up to 3,000 sq mi in size, corresponding to the trend shown in Fig. 1. A number of companies, including Marathon, Sonat, Union Pacific Resources, and Zackson Resources now have multirig drilling programs in progress, and others will follow soon.

As of July 1, 1997, some 25 discoveries had been reported out of 45 wells drilled on reef targets, for a 55% success rate (Fig. 1). Initial rates of production have varied from 4.3 MMcfd (Zackson 1 Manzikert) to as high as 57.5 MMcfd (TXO 1-X Marshall A), with estimated ultimate reserves for single wells of 5-80 bcf. A minimum of about 550 bcf of reserves have been discovered thus far.

Depth to the top of productive reefs is in the range of 13,000-15,000 ft TVD. The combination of small reef size, steep flanks, and migration complexities makes these features difficult to image and locate precisely, with the result that a large number of wells have been sidetracked.

Reservoir porosities range from 7-30%, over intervals of 50-300 ft, and include both primary and secondary types. Permeability data remain mostly proprietary. Dolomitization of reefs is minimal to absent.

The following report presents an introduction to the basic geology of the reef play. Debate exists among industry geologists as to the precise nature, origin, and occurrence of these reefs, which are characterized by considerable diversity. An attempt is made here to include mainly information about which some degree of consensus exists.

Brief play history

Oolite shoal facies of the Cotton Valley Lime/Haynes- ville interval have been productive since the 1950s along the western shelf of the East Texas basin.

During the early 1980s, several companies, including Texas Oil & Gas (TXO), extended discovery in this facies downdip to the shelf/ ramp margin in northeastern Leon and southern Freestone counties. TXO drilled several wells that encountered abnormally high pressures and rates of gas flow from non-shoal carbonates encountered several hundred feet above the projected top of the Cotton Valley Lime. In one instance (Branton field, Leon County), TXO identified a "pinnacle" type feature.1 However, core data were lacking to confirm a reef interpretation and thus a potential new play.

The current play began with the drilling of the 1 Poth by Marathon Oil in 1993, approximately 2 miles south of the Branton field discovery (1 Marshall). By acquiring TXO in 1989, Marathon inherited crucial well and seismic data suggesting the occurrence of multiple pinnacle reefs along a faulted Cotton Valley Lime shelf/ramp margin.

Using this basic model, the company identified a series of prospects, mainly in Leon County, and initiated drilling with the 1 Poth, tapping the largest gas reserves yet estimated for any reef feature (110 bcf). Between 1994 and 1995, Marathon extended the play by drilling an additional six wells with three new discoveries (1 Nash, 1 Riley Trust, and 1 Broun- kowski).

As of 1996, drilling increased significantly (Fig. 1), due to new companies entering the play. Out of a total 18 wells in 1996, 11 discoveries resulted (61% success rate). During the first 7 months of 1997, a total of 30 wells had been drilled, spudded, or were under evaluation.

Six discoveries and 10 dry holes were reported as of July 1: This lower rate of discovery (37.5%) should be weigh- ed against the fact that 14 of the 16 reported wells hit reef targets (87.5%), indicating the considerable success of 3D seismic data in identifying actual reef features. Several of the unsuccessful wells encountered impermeable reefs, and several more missed the reef core and are being sidetracked.

Regional geology

The East Texas basin is an interior salt province characterized by gently dipping shelves rimming a central depocenter (Fig. 2 [36,013 bytes]). To the west and north, the basin is bounded by the Mexia-Talco fault zone, a major tectonic belt of normal faulting that roughly overlies the buried Ouachita thrust front and corresponds to the updip limit of Louann salt. On its eastern and southern sides, the East Texas basin abuts the Sabine uplift and Angelina-Caldwell flexure, both of enigmatic origin.

Low and high-relief salt swells and salt anticlines underlie shelf areas of the basin, while piercement domes of varying size and geometry exist downdip, punctuating the outermost portions of the shelf and the deep basin proper.

Recent paleo-reconstructions indicate that the East Texas trough was divided into a complex series of minibasins and intervening basement-cored platforms during the Late Jurassic. Salt movement began as early as Norphlet/Smackover deposition, producing low-relief salt anticlines, with local associated faulting by Cotton Valley Lime time.

Data shown by Jackson and Seni2 suggest that the major phase of salt movement began in the south (Late Jurassic-Early Cretaceous) and progressed northward. As a result, structures observed today along the shelf do not necessarily reflect those active at the time of reef growth.

Stratigraphically, the Cotton Valley Lime (also known as "massive lime") represents the downdip, marine equivalent to the mainly non-marine, clastic Haynesville formation. The major portion of the Cotton Valley lime consists of lime mudstones and wackestones, peloidal packstones, and oolitic and skeletal grainstones. These rocks overlie the Smackover and Buckner interval and underlie thick, dark-colored shales of the Bossier formation (Fig. 2).

Reef occurrence is restricted to the uppermost portion of the Cotton Valley lime and overlying lower Bossier shales, which encase the major portion of most reef bodies. Capping the Bossier over much of the basin is the Knowles limestone (Latest Jurassic-Early Jurassic), also known to contain reefal material in some locations.3

Occurrence, character

Steep-sided, pinnacle-like reef bodies extend upward as much as 500 ft or more into the lower Bossier shales, with which they are mainly time-equivalent.4

Reefs occur over a distance of up to 15 miles or more in the dip direction, along the middle and outer portions of the paleo-ramp in Leon and Freestone counties. In more updip areas, the character of seismic anomalies suggests a change from pinnacle morphology to patch-type reefs.

In southern Robertson County, increased clastic and fresh-water influx, apparently related to a basinward swing in the paleo-shoreline, resulted in reef poisoning and general loss of coral-rich facies. Features in this southern area appear to be dominated by sponge-thrombolite facies, in contrast to more coral-rich features to the north.

Seismic data suggest reefs grew in a wide range of specific settings, as indicated by the cross section of Fig. 3 [16,689 bytes]. This cross section is based directly on a regional seismic line through Freestone and northern Leon counties.4 No less than six individual reef-type anomalies are identified on this line, the most updip of which occurs in the vicinity of Teague Townsite field, 10 miles northwest of the main reef drilling trend.

Core data, well logs, and comparison with other Upper Jurassic reef occurrences worldwide (e.g., Leinfelder5), indicate that that reefs are stacked features composed of multiple facies units, each representing a different episode of growth or drowning. Dominant reef builders consist of corals, calcareous and siliceous sponges, and calcareous al- gae, with accessory biota such as chaetetids, gastropods, pelecypods, echinoids, encrusting forams and bryozoans, and other species.

A significant component in many reefs is microbialite, consisting of microbe-produced micrite that acts as a crucial binding agent. Microbialite often displays thrombolitic (clotted) textures and, in its capacity to bind together heterogeneous debris produced by bioerosion (e.g. boring sponges), wave erosion, and other processes, was required for rapid upward growth and the considerable height these reef features were able to attain.

Two main facies groups exist: coral-rich and sponge/ thrombolite-rich, both of which exhibit diversity in specific lithology, biota, and thickness. Coral-rich types reflect higher energy settings and can include framestone and microbolite-debris subtypes. Sponge/thrombolite-rich facies reflect more quiet-water conditions, either in deeper or more protected settings, and tend to be microbialite-dominated and impermeable.

Individual reef bodies display complex stacking of both facies groups, implying a changing response to sea-level fluctuations. Most productive reefs have zones characterized by significant dissolution and recrystallization. These zones are interpreted as the result either of subaerial exposure or post-burial fresh-water invasion.

It is difficult to decide between these two models on the basis of observed textures alone. The presence of local grainstone and oolite facies, plus rare quartz pebbles, argues for exposure. However, the lack of cores, either whole-rock or rotary sidewall, has hampered detailed interpretations of reef growth and character.

In addition to differences in depositional setting, these reefs have undergone varied and complex diagenetic alteration. Dissolution and recrystallization, widespread in some zones, have been interpreted as the result of intermittent reef exposure during deposition or later introduction of fresh water. Further, the effects of burial diagenesis are significant, including extensive cementation by non-ferroan and ferroan calcite, growth of authigenic illite, lining and reduction of porosity by pyrobitumen, and emplacement of sphalerite, barite, and other minerals.

Reef growth

The combined evidence of core, log, and seismic information supports the interpretation, established for Upper Jurassic reefs in Europe, Africa, and elsewhere, that growth was directly related to sea-level fluctuations, in particular to fourth-order transgressive/regressive cycles (Fig. 4 [43,816 bytes]).

Reef nucleation appears to have taken place during the early (transgressive) phase of each cycle, with subsequent accommodation ("catch-up" growth) during continued sea-level rise. Upward growth in many cases continued to wave base or higher; during the succeeding regressive portion of the cycle, the reef crest was sometimes exposed or subjected to wave erosion.

Re-working of resulting debris and re-colonization by reef biota marked initiation of the next cycle. Particularly rapid transgressive episodes resulted in partial drowning, reflected in transition from coral-rich or mixed coral-sponge facies to more siliceous sponge-rich/thrombolite facies.

Cycles of sea-level rise and fall took place against a background of continuous third-order transgression. Back-stepping of reef nucleation and facies development is suggested, whereby successively more updip portions of the paleoramp became the sites for reef initiation and, later, deeper water facies development.

Thus, for example, the lowermost facies unit (growth zone) of one feature may correspond chronologically to the second or third facies unit of a more downdip feature. In general, features with abundant coral-rich facies developed in unprotected settings on the flanks of salt highs, along faulted margins, horst blocks, and on the upthrown sides of active or incipient faults that dip both basinward and counter-regionally. More sponge-dominated features appear characteristic of protected conditions, such as existed in back-shoal areas or in more outer ramp settings.

In southern Robertson County, a southeastward swing in the paleo-shoreline appears to have caused increased fresh water and clastic influx, confining reef growth to depths below the freshwater lens, thereby preventing exposure and porosity development.

It must be emphasized that Fig. 4 represents a highly simplified model of reef growth. Primary determinants on reef development included water depth/energy, background rates of clastic input, water salinity, and oxygen/nutrient levels, all factors that may have varied locally. Regional reconstructions have emphasized that western portions of the ramp/shelf are more likely to have been exposed to waves and ocean currents arriving from the southeast, with the rest of the East Texas embayment sheltered by the Sabine uplift. This interpretation, however, has more recently been rejected by some workers, who stress that reef growth took place within a highly dynamic depositional system involving both sea-level fluctuations and tectonic movement.

Seismic character

Over most of the mid- and outer-paleoramp, the Cotton Valley lime is associated with a pair of positive reflectors separated by about 35-40 ms (Fig. 5 [34,931 bytes]). Reef features are confirmed by drilling display dim-out of the upper reflector and, at times, the lower reflector as well. Some proven reefs also exhibit small positive events over their crest.

Drape is observed in the overlying Bossier shale, where reflections exist. Lower Bossier events, if conspicuous, onlap the paleo-ramp at positions corresponding to reef occurrence, implying reef growth was controlled by sea-level transgression.

Due to the small size and significant depth of reef features, data migration must be performed carefully. Look-ahead VSPs (vertical seismic profiles) are often run during drilling in order to accurately identify reef location but have not proven helpful in all cases. Analysis of seismic data has been unable to distinguish porous from non-porous reefs.

Reservoir data

Cotton Valley Lime reef reservoirs display significant variation in reservoir character and quality.

Both primary and secondary porosity types exist. Primary types include framework, intraparticle, and shelter; secondary porosity includes vuggy, micro-intercrystalline, moldic, and fracture types. Of these types, vuggy and micro-intercrystalline pore systems seem to contribute most to reservoir quality, though framework and fracture types are locally important.

All reefs show evidence of early-stage primary and moldic megaporosity; to varying degrees, such megaporosity was later filled by ferroan calcite, which fluid inclusion analyses suggest formed during deep burial, at temperatures of 250-300° F. As shown by the log of Fig. 6 [32,000 bytes] from the 1 Brounkowski (northeastern Robertson County), porosity tends to occur in zones separated by thin, nonporous intervals. Such zonation is assumed to be related to stacking of facies units controlled by cyclic sea level fluctuations.

Using a cutoff of 7% porosity, Fig. 6 indicates potential reservoir zones varying from 6 ft to 80 ft thick, with values ranging from 7-24%. Some wells, such as the 1 Sherrod and 1 Noey (Leon County), have shown low or poorly connected porosity but were successfully frac'd.

Controls on permeability are not well understood. Excellent permeabilities have been observed in core samples exhibiting pervasive recrystallization of original microbiolite-bound debris facies. Fracturing may also play a more significant role than previously believed.

The best reservoirs are overpressured, with bottomhole shut-in pressures of 11,500-14,000 psi at depths of 14,100-15,000 ft. Mud weights of up to 17-18 lb/gal have been used to maintain well control. High pressures make significant production rates possible at porosities as low as 6-8%.

Total reserve estimates range from 0.5-60 bcf (average 15-20 bcf) for single wells and up to 80-110 bcf for single reef bodies (e.g. 1 Blanton, 1 Poth).

The ubiquitous presence of bitumen in productive facies suggests that early charging of reefs with oil was succeeded by increased burial/heating, cracking, and in-place gas generation, leading to overpressuring. Potential source rocks include the underlying Smackover and the Bossier.

Play outlook

Currently, the East Texas reef play is in a critical phase with regard to future development.

Recent drilling results showing a lower rate of success, particularly during late 1996 and the first half of 1997, must be viewed in context. This context includes the following factors:

This period has experienced a highly increased pace of drilling, which, in the history of any frontier play, is fated to result in a greater number of dry holes. This is especially true of a play in which seismic data are able to locate relevant drilling targets but unable to distinguish their prospectivity.

Four of the 10 dry holes drilled in the first half of 1997 are located in southern Robertson County and have helped delineate this portion of the trend as being more dominated by tight reef facies and by non-reef anomalies (slump features?). As noted, proximity to the Bossier paleo-shoreline is believed responsible for this.

Only two wells out of 16 in the first half of 1997 failed to encounter reef facies. It is clear that improved geologic understanding is required to predict the occurrence of porous vs. non-porous reefs. Porous reef features may be restricted to those that grew in shallow, basinward settings in bathymetrically elevated positions.

A significant number of recent wells have been reportedly located on second-order anomalies, or anomalies that do not suggest reefs with highest potential. The reasons for this are not clear but may involve considerations related to lease ownership. The result is that several or more first-order anomalies remain undrilled within the main fairway.

Prospecting for Upper Jurassic reefs in East Texas thus remains a challenging exploratory venture with high, if qualified, upside potential. At current rates of per-well discovery (average 25 bcf per well at a 37.5% success rate), each additional 50 wells should bring a minimum of 470 bcf added reserves. With hundreds of potential reef locations already identified, this implies a potential total resource of several trillion cubic feet of gas or more.

Challenges continue to exist with regard to geological and geophysical interpretation, as well as drilling and completion. At present, the drilling fairway encompasses less than 20% of the total prospective trend. Anomalies similar to those in Freestone and Leon counties have been identified as far north as Wood County, along the east side of the basin, and also within the basin itself, along the margins of basement-cored platforms and salt structures (Fig. 1).

It appears likely, therefore, that reef drilling will continue to revitalize exploration in the East Texas basin, a province that, only a few years ago, was said to have already yielded its best.

References

  1. Montgomery, S.L., Cotton Valley Lime pinnacle reef play: Branton Field, AAPG Bull., Vol. 80, No. 5, 1996, pp. 617-629.
  2. Jackson, M.P.A., and Seni, S.J., Atlas of salt domes in the East Texas Basin; University of Texas Bureau of Economic Geology, Report of Investigations 140, 1984, 102 p.
  3. Finneran, J.M., et al., Lowermost Cretaceous ramp reefs: Knowles Limestone, southwest flank of the East Texas Basin; in Jurassic of the Gulf Rim, Gulf Coast Section, SEPM, 1984, pp. 125-133.
  4. Montgomery, S.L., Upper Jurassic reefs, East Texas Basin: stratigraphy, depositional models, and petroleum potential; Petroleum Frontiers, Petroleum Information Corp., Vol. 13, No. 4, 1997, 68 p.
  5. Leinfelder, R., et al., Paleoecology, growth parameters and dynamics of coral, sponge, and microbolite reefs from the Late Jurassic; in Reitner, J., et al., eds., Global and regional controls on biogenic sedimentation, Göttingen: Göttinger Arb. Geol., Paläontologie, Sb2, 1996, pp. 227-248.

The Authors

Scott L. Montgomery is a petroleum consultant and author residing in Seattle. He is the lead author of the "E&P Notes" series in the AAPG Bulletin and the quarterly monograph series "Petroleum Frontiers" published by Petroleum Information/Dwights LLC. His current research interests include frontier plays and field redevelopment in North America. He holds a BA degree in English from Knox College and an MS in geological sciences from Cornell University.

Gus Wilson has been a Houston carbonate geology consultant since 1982. His professional experience includes extensive work in exploration in the U.S., Saudi Arabia, and the Middle East generally, with particular focus on Mesozoic reservoirs. He holds a PhD from the University of North Carolina and a BS from the University of Tennessee.

Daniel G. Ziegleris an exploration geologist with SK Resources, Houston, a major player in the current East Texas Cotton Valley Lime reef play. Before joining SK in 1991, he worked as a geologist with Southeastern Exploration and Production, focusing on play potential of the eastern U.S. Triassic rift-graben trend. He holds a BS in geology from Indiana University of Pennsylvania and performed graduate work at the University of Texas, Dallas.

Keith Hatchhas been vice-president of exploration for Sigmund, Kane & Hatch Inc., Houston, since 1990. He has worked previously for Texaco, North Central Oil Corp., and British Petroleum. He has a BS degree in geophysics from Penn State University.

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