Thomas E. Ewing
Frontera Exploration Service
San Antonio
W. Grant Fergeson
Venus Oil Co.
San Antonio
The Downdip Yegua trend of overpressured gas-condensate reservoirs has produced over 600 bcf and 15 MMbbl of oil and condensate, mostly since 1979. The trend has indicated reserves in the range of 1.6-2.0 tcf, and substantially higher potential.
The trend was opened only in the late 1970s and 1980s because its sandstone fairways are for the most part separated from updip sandstone-rich Yegua section (which has produced since the 1930s) by a "mid-dip" region where sandstones are scarce. The trend is also separated from overlying Frio and Vicksburg targets by over 1,000 ft of highly overpressured Jackson shale.
Five fairways can be defined:
I. The Duval-Jim Wells fairway, with thin but continuous sandstones and little expansion of section;
II. The Wharton-Jackson fairway, with thick but discontinuous sandstones of high quality and very active faulting and expansion;
III. The Fort Bend-Brazoria frontier, with deeply buried sandstones affected by growth faulting and salt movement;
IV. The Harris-Jefferson fairway, with abundant sandstones, no mid-dip region separating it from updip deltaic sediments, abundant salt structures, and growth faulting;
V. The Orange-Calcasieu fairway, with widespread sandstones but a narrower belt of growth. faults and traps.
Fairway IV has the most production to date (372 bcf), but it was also one of the earliest to be developed (beginning in 1953). Fairway II has produced 164 bcf since its discovery in 1982, and is the most active fairway to date. Fairway I has produced 70 bcf since 1979, and Fairway V has produced in the vicinity of 11 bcf. Fairways III and V are in the least mature stage of development.
Sandstones in the Wharton-Jackson fairway are fed by dip-elongate channel systems which extend through the "Mid-dip" region; these channels are presently a focus of exploration for normally-pressured gas. Downdip, the channels feed a variety of deltaic, bar/strandplain and shelf environments. The episodic deposition downdip from the main Yegua (Katy) delta system is due either to episodic lowering of eustatic sea level, or to infrequent progradation of narrow birdsfoot deltas into the middle and outer shelf.
At present, activity is mainly concentrated on detecting "sweet spots" of a few hybred acres or less; evaluating downdip and lateral extensions of known trends; and in defining large, potentially prolific structures in frontier areas using integrated geologic and geophysical analysis.
The Downdip Yegua trend--defined as the zone of overpressured gas-condensate production from Yegua sandstones -is a set of rapidly developing and proven prolific exploration fairways in the Texas-Louisiana Gulf Coast.
It is a rarity in recent history of Gulf Coast exploration: a sandstone fairway at moderate depth, which was hidden below overpressured Jackson shales, and whose presence was largely unsuspected before the late 1970s and early 1980s.
The presence of thick sandstones miles downdip of the known Yegua sandstone depocenters was not predicted by previous facies models.
REGIONAL SETTING
The general facies assemblages present updip are shown on Fig. 1. In Southeast Texas a fluvial-dominaed delta complex here called the "Katy Delta system" built southward into the Houston diapir province, depositing thick sand accumulations north, east and west of the city of Houston. These sandstones have produced over a billion bbl of oil and 300 bcf of gas in large salt-related structures such as Conroe, Tomball and Katy. This trend was opened in 1931 at Conroe field; a discovery of significant fields continued into the early 1950s. At its downdip edge, the Yegua interval was known to become overpressured (at Clinton field in Harris County and Raywood field in Liberty County).
To the east of the Katy Delta system, Yegua sandstones persist eastward along the Wilcox fault trend in Hardin, Tyler, Jasper and Newton counties and Beauregard Paris, La. These sandstones were deposited in a strandplain and/or wave-dominated delta environment over the southern flank of the Sabine arch. Significant fields such as Silsbee, Nona Mills, Segno and Sabine Tram were found in the 1940s and 1950s.
West of the Katy Delta system, a strandplain and/or barrier complex continued westward along the Wilcox fault trend in Wharton, Colorado, Lavaca and Dewitt counties. Yegua production frequently overlies deeper Wilcox production, as at Garwood/Provident City, Nada, Chicolete Creek and Brushy Creek. This area was actively developed in the 1940s and 1950s, in part concurrent with Wilcox development. Downdip of this play, sandstone thickness and percentage decreases rapidly, although fields were developed to within a few miles of what we now know as the Downdip trend (Welder Ranch, Spanish Camp and New Taiton).
Strandplain and/or barrier sandstones continue south-westward across the San Marcos arch through Goliad, Bee and Live Oak counties. In the Live Oak and Duval County area, increased sandstone percentages and updip channel feeders indicate that a wave-dominated delta system fed into the strainplain in this area. South of northern Duval County, the Yegua contains strike-oriented bar and strandplain sandstones, many of which produce oil along their updip pinchouts.
THE FIVE FAIRWAYS OF THE DOWNDIP TREND
Development of the Downdip Yegua overpressured trend allows us to distinguish four fairways for Yegua sandstone development and exploration.
DUVAL-JIM WELLS FAIRWAY
This southernmost fairways lies between Sejita dome in southeasternmost Duval County and the Orange Grove area of northeastern Jim Wells County (Fig. 2). In this area, thin sandstones of Upper Yegua and Lower Yegua age which are continuous over most of the fairway represent the downdip edge of the wave-dominated delta system defined to the northwest. Growth faults in this area have only a slight effect on sandstone development or interval expansion; they may be reactivations of older (Queen City?) structures.
The fairway was first recognized as a significant gas trend in the late 1970s, with discovery of Alice Deep field and the ASOG-Palito Blanco complex both of which were overpressured and yielded and excellent tests. (The Sejita field to the south has produced Yegua gas for some time, but was normally pressured.) Subsequent drilling has tested gas both to the north (Caro Creek, Muerto Creek, Muy Solo) and south (Jaboncillos, Orcones, Glasscock Ranch) of the ASOG field complex which has been extended updip into the Four Sevens and Starr Brite, West fields.
Recent exciting tests by Coastal Oil & Gas at Ben Bolt, West field have opened a slightly downdip extension of this Yegua production. The field-opening 1 Hoffman GU, flowed 10.4 MMcf/d and 177 bc/d on adjustable choke from 7,622-7,776 ft with a BHSIP of 6,377. The 2 Hoffman produced 2.6 MMcf/d from a different sandstone.
Total production to mid-1988 totals 70 bcf gas and 1.1 million bbl of condensate (MMbc); reserves are about 200 bcf.
So far, none of the very few deep tests in the area immediately updip of the Vicksburg fault zone between Orange Grove and Inez field in easternmost Victoria County have encountered overpressured Yegua sandstones. This area lies downdip of the Yegua strandplain-barrier system which crosses the San Marcos arch in this gap, so possibly sand influx was too limited. However, indications of Yegua channel sandstones in northern Goliad and Victoria counties led to outpost wildcats, such as the Sun 1 Miori (D&A Ca, 15,000 ft).
JACKSON-WHARTON FAIRWAY
The most exciting part of the Downdip Yegua trend lies between Inez field in Victoria County and Pierce Ridge field in Wharton County, (Fig. 3). In this area, prolific production has been obtained from thin to very thick sandstones of excellent quality (100-1,000 md, up to 30% porosity) although frequently laminated. The sandstones are variable in all directions, displaying a complex of shallow-water depositional environments from deltaic to barrier-bar and channel (with shelf(?) turbidites. Active growth faulting during deposition contributes to stratigraphic and structural complexity.
The trend was discovered by the Tartan 1 Menefee test in Wharton County in 1981, drilled as a rank wildcat to the Yegua and discovering 150 ft of sandstone. An ultradeep Wilcox test in Jackson County, the Shell 1 Drushell, discovered highly porous and permeable sandstones in that part of the fairway. Intense seismic exploration led to the discovery of Black Owl field by Owl and Blackjack in 1982, Toro Grande by Cities Service in 1984 and Shanghai by Ladd Petroleum in 1985. Exploration continues to be very active and successful through 1988.
The updip boundary of the sandstone-bearing trend is very sharp in most places, sandstone-free wells have been drilled within 4,000 ft of wells containing over 1000 ft of sandstone. Farther updip, prolific normally-pressured gas wells have been brought in (Rio Loco and other fields), producing from narrowly confined channels which are inferred to represent feeders for downdip deposition.
In Wharton County, sandstone occurs in four intervals: the Upper Yegua DY (Discorbis yequaensis), Upper EY (Eponides vequaensis), Basal EY and the Lower Yegua AU (Anomalina umbonatus), (Fig. 4). The AU has produced from discontinuous sandstones in Phase IV and Gresham field. The Basal EY produces from a local delta and associated bar system in Phase IV, Gresham, Black Owl and Shanghai, East field. The Upper EY produces from a thick strike-elongate bar sequence (up to 1,000 ft thick) in Black Owl and Gresham field. The DY produces from a local delta and distal delta-front sandstones and Shanghai, Shanghai East and El Campo field. This last sequence is the "Shanghai Delta trend."
A large domino-style gravity-slid system is easily recognized on seismic data throughout the fairway; it affected the entire Yegua sequence, but appears to have been mostly of Upper EY and DY age. Most of the productive sandstones and structures have to date been associated with a complex of faults immediately updip of the gravity slide. A large gentle domal high cut into many segments forms Toro Grande, the premier field of the Downdip Yegua trend with up to 200-300 bcf reserves.
Production to mid-1988 totals 165 bcf of gas and 3.9 MMbc. Of this, 115.6 bcf and 1.5 MMbc comes from the Jackson-Victoria group of fields (dominated by Toro Grande, with 96 bcf), and 49.3 bcf and 2.4 MMbc comes from Wharton County (mostly from Shanghai, Black Owl and El Campo, with 194. bcf, 11.6 bcf and 12 bcf). Reserves found to date are in the vicinity of 250-300 bcf in Wharton County and 400 bcf in Jackson and Victoria counties.
Exciting new field discoveries promise that these figures will increase substantially. The Ladd 1 Run opened Menefee field in Wharton County, flowing 2.4 MMcf/d and 27 bc/d on a 8/64 inch choke at 7,610-20 ft. Four wells now produce from Phase IV field, which was discovered in 1987 by the Cox & Perkins 1 Gold-Zapp, which flowed 4.1 MMcf/d and 452 bc/d on a 12/64 inch coke from a depth of 10,506-45 ft (BHSIP of 8,975 inches). In Jackson County, Jack Starr field was opened with the Atapco 1 Labay, which flowed 1.3 MMcf/d on 11/64 inch choke from 8,041-8,071 ft (BHSIP of 6,518 ft); Coastal Oil and & Gas opened the Cordele East field with the 1 Harrison, flowing 1.8 MMcf/d from 7,840-86 ft; and Ken Petroleum opened the Kenlon field with the 1 Melton unit, flowing 2.7 MMcf/d from 9,391-95 ft.
FORT BEND-BRAZORIA FRONTIER
The area east of the Wharton-Jackson fairway has seen sparse drilling and few discoveries to date. The area lies within the Houston diapier province, and all structures are somewhat salt-affected. The Yegua expansion system occurs farther downdip than in Wharton County; a few marginally overpressured and unexpanded wells have been drilled in central Fort Bend County (including the Arco 2 George Foundation, discovery well for the one-well Booth field in 1981).
Two significant discoveries have been made in this segment of the trend. The Venus 1 Dutton discovered a thin sandstone in the Basal EY at 14,656-67 ft, opening the Needville, South field in 1987. The Dutton flowed at 1.13 M Mcf/d and 35 bc/d at a BHSIP of 11,916 from 6 ft of net pay. The well is on a deep salt structure, which also localized shallow Frio production. Substantially thicker sandstones within the same interval have also been found to the west in Inexco 1 Dutton and the Ashland 1 fields tests. Significant Yegua sandstones have also been found far basinward in the Arco 1 Crewes and the TXO 1 Johnson wells in Brazoria County.
To the east, Ladd Petroleum has drilled a significant test, the 1 Sharp, in Brazoria County, testing gas briefly from several hundred ft of sandstone found below 15,000 ft. Thin sandstones are found north of this test in the Cenergy 1 IP Farms and the Arco 1 Fuqua. Followup drilling in this area has so far failed to establish production; but activity remains intense, due to the encouraging results from the Ladd 1 Sharp.
HARRIS-JEFFERSON FAIRWAY
This overpressured sandstone fairway is the direct downdip continuation of the Katy delta system. Exploration in the overpressured environment has been going on since the mid 1970s, resulting in many field discoveries. Three groups of fields are most prominent; the Ulrich-Clinton cluster, the Raywood-Rich Ranch cluster, and the Jefferson-Hardin cluster (Fig. 5). Total production has been 373 bcf and 9.3 million bbl of liquids.
The cluster of fields with the most significant recent activity lies in eastern Harris and adjoining counties, centered on Ulrich field, discovered by HNG (now Enron) in 1984 and developed by Enron and Mobil. This field has produced 41 bcf to mid-1988, and may have over 75 bcf of reserves. The excellent productivity of Ulrich reservoirs, reminiscent of those in Wharton and Jackson counties, has sparked an intense search in this area. The American Hunter 1 Texas Northern RR, opened Canadian Hunter field with a flow of 4.9 MMcf/d on 14/64 inch choke from Upper Yegua at 11,640-666 ft.The old established production at Clinton field and on the flanks of Barbers Hill also fall within this cluster. Production is from the flanks of a salt ridge, and from various growth-fault related structures. Production to date from the cluster has been 127 bcf (mostly from Ulrich, Clinton and Barbers Hill) and 2.2 million bbl of liquids (condensate an oil).
In Liberty County, a group of field clustered around the old Raywood (1953) and Rich Ranch (1960) fields have been developed between 1976 and 1985. The fields lie on the rim of a deep salt-withdrawal basin between Hull and South Liberty domes and downdip of growth faults which cross a salt-cored high southeast of Hull Dome. The area lies immediately downdip of normally-pressured oil and gas production at Hull and South Liberty salt domes. Total production has been 148 bcf of gas and 2.7 million bbl of liquids.
The eastern cluster, in Jefferson and Hardin counties, is dominated by Sour Lake East (1978), and Constitution (1979). Production is mainly from Lower Yegua sandstones over faulted salt-cored ridges. The faults show expansion and rollover in the Lower Yegua sequence. The downdip part of the cluster contains enhanced Upper Yegua sandstone in a separate fault system. Production from this area has been 88 bcf gas and 4.4 million bbl of liquids. A recent fine discovery has been made by Quintana, opening Big Prairie field with the #1 Carpenter. This well flowed 5 MMcf/d and 768 bc/d from perforations in the Upper Yegua at 9,461-74 ft, and yields a calculated AOF of 575 MMcf/d.
ORANGE-CALCASIEU FAIRWAY
The easternmost fairway of the Downdip Yegua trend to data extends eastward from Orange and Newton counties, Texas through Calcasieu Paris, La. (Fig. 6). This fairway lies just outside the Houston and South Louisiana diapir provinces, along a fault system which was earlier known from Frio production. Similar to the Wharton-Jackson area, the area immediately updip of the fairway is sandstone-poor, although less drastically so.
The fairway contains two subareas. The Orange County segment contains distinct Lower Yegua and Upper Yegua fault systems similar to the Jefferson-Hardin cluster to the west. An early test, the Gulf Miller-Vidor Unit 3, produced 9,000 bbl of oil from an Upper Yegua sandstone in 1960. Later exploration has had Lower Yegua objectives, resulting in Long Prairie, Gist SE and Lemonville fields in 1979-1985. Production of mid-1988 has totalled 3.8 bcf and 191,000 bbl of liquids.
The Louisiana segment is characterized by a single growth fault system of great length, which runs from a structural transition in Newton County to Jefferson Davis Paris. Elongate rollover anticlines and wedges down-thrown to the fault have been produced at Perkins, East; Perkins, South; and DeQuincy, West. Exciting new tests have been drilled near the Sabine River, Prairie Producing has opened Starks, West field with its 1 Hankamer, producing over 1,000 bo/d and 8.9 MMcf/d from an Upper Yegua(?) sandstone at 10,940-11,102 ft ( BHSIP of 9.243 ft); two equally prolific offsets have been drilled. Nearby, Maralo has discovered a new field with the #1 lies, with production of over 300 bo/d and 340 Mcf/d.
In summary, the Downdip Yegua trend has produced over 618 bcf of gas and nearly 15 million bbl of oil and condensate, mostly since 1980. Discovered reserves probably exceed 1,600 bcf to date, and active exploration will increase this number to well over 2 tcf.
At present, prospecting activity is mainly concentrated on detecting "sweet spots" of a few hundred acres or less within known fairways, using AVO analysis of seismic data in conjunction with detailed geological and geophysical mapping; evaluating downdip and lateral extensions of known fairways and in defining large, potentially prolific structures in frontier areas using integrated geologic and geophysical analysis.
In the Wharton-Jackson and Orange-Calcasieu fairways, substantial amounts of sandstone and shale containing beach and inner neritic fauna were deposited downdip of a muddy shelf. Occasional channel sandstones up to 60-100 ft thick are penetrated by wells in this "mid-dip" area; some of these produce gas, as at Welder Ranch, Jones Creek and Rio Loco.
These areas were hidden from earlier discovery because of the generally sand-poor character of the "mid-dip" shelf area.
An overall sandstone isopach shows that the major deltaic and strandplain depocenters roughly overlie the Wilcox fault system (the Katy Delta and related strandplains of Fig. 1); local sandstone accumulations occur miles downdip of these depocenters, fed by dip-oriented channel systems. How does this happen?
Two possibilities can be entertained. In one, eustatic drops in sea level cause incision of a muddy shelf by streams which rework the updip highstand sand systems and deposit sediment on the outer shelf or over the shelf edge as point-source deltas which were subsequently reworked along strike.
In the other, climatic changes or some other infrequent cause allowed rapid progradation of a "birdsfoot" delta and/or related shelf turbidites over the muddy shelf to the shelf edge. This delta was subsequently abandoned and the usual updip deltaic sedimentation resumed. The possibility of hurricane-generated turbidity currents contributing to sand sedimentation has been raised. Deltaic sedimentation must have been very rapid to avoid reworking into a lobate form by Gulf waves and currents.
The exploration implications of this unresolved question are significant. If downdip sedimentation is eustatically controlled, sandstone bodies of the same ages (and possibly similar qualities) as those in Wharton County can occur anywhere along the trend; and in the main delta complex reworked zones of high quality could be found. If, however, birdsfoot deltas are responsible, they should be limited to the areas with updip deltas and possibly represent different ages in different areas.
These facies models also contain implications for other clastic sequences within the Gulf Coast basin. If the relatively minor Yegua formation deltas could achieve transport and deposition of such tremendous quantities of high-quality sand farseaward of the primary deltaic deposcenters, what does this imply for other systems, which are not known at present to have an overpressured or shelf-edge fairway associated with them?
ACKNOWLEDGMENTS
We thank Bonnie Weise (Venus Oil Co.), for assistance with data from Fairways IV and V; Diane McDaniels, for compiling data for Fairway 1; and Venus Oil Co. itself, for permission to publish.
The authors have references.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
