TOOLS ASSIST IN MAPPING FRACTURED RESERVOIRS

Santiago M. Reynolds
Houston

A fracture is any break in the earth's crust, whether or not it causes displacement due to mechanical failure by stress. Fractures include cracks, joints, and faults.

Usually thousands of fractures are associated with each fault (Fig. 1).

Fractures cut through all lithologies: consolidated rocks, sands, clays, and soils. All fractures are partially horizontal and vertically healed and concealed. The softer the rock or soil, the easier to heal and conceal the fractures, though their identification is still possible. Regional fractures tend to be linear, often extending for miles. These long fractures are harder to map since they are normally concealed by soil. Consequently their surface trace is interrupted along their course and must be extrapolated between visible fracture signatures.

A fracture segment can be represented by drainage courses, but not all drainage represents fractures. Local fractures can be both linear and curve and for all practical purposes less than 2 miles. Faults are easier to identify, trace, measure, and describe than fractures.

The Gulf Coast and other areas with similar deposits are highly faulted and fractured. In a given area it is possible to have faults and abundant fractures or only fractures but not faults without fractures. Most of the fractures are still active in the same way that fractures continue to form on man-made covers such as buildings, streets, and highways. Eventually they show up on the surface.

Fractures are the media through which fluids can travel.

Most of the vertical fractures show up on the surface allowing fluids like oil to accumulate in seepages and gases to be deposited in soils where their identification is possible by geochemical analysis.

The same influence prevails for radiometric, micromagnetic, and other less known surveys.

GEOMORPHICS DEFINITION

Geomorphics may be defined as the science that treats the general configuration of the earth's surface, especially the study of the classification, description, nature, origin, and development of present land forms and their relationship to underlying structures, and of the history of geologic changes as recorded by these surface features.

It is the study of land forms and the forces that cause specific forms to develop. Relief details show differences that lie beneath the surface. Land forms show the imprint of internal earth movement, erosion, and deposition."

GEOMORPHIC STRUCTURAL FACTS

Thousands of wells prove that geomorphic structures can represent deep-seated structures. An exception is the overthrusted or mountainous terrain.

In other cases it has been corroborated that drainage patterns by themselves are delineating deep-seated structures beneath flat surfaces when several wells were drilled on such proposed structures. In other instances a large geomorphic structure has been delineated that also represented a thickening of one of the subsurface gas reservoirs, indicating a classical stratigraphic wedge.

Once again, mapped subsurface data from wells proved the interpretation. In mountainous terrains, these maps can usually aid understanding of geologic features such as faults, fractures, large and small structures, and different lithological influences.

In other structurally complicated areas where angular unconformities or stratigraphic traps are known, these maps can be of great assistance if a relationship can be established between producing wells and their geomorphic expressions. New areas of interest can be established by analogy.

A pilot area was mapped across Beauregard Parish, La., and Newton County, Tex., along the producing "8,100 ft sand" in the Yegua trend. Structural relationships were established between the geomorphic structural interpretation and the subsurface mapped producing fields. What was needed was to migrate the geomorphic structures to the production horizon by sliding the overlay until a possible match of both maps became apparent. This technique opens new horizons for Gulf Coast operators that need to extend any producing field where subsurface data or seismic lines are scattered or not available.

FRACTURE APPLICATIONS

Oil companies have conducted research towards a better understanding of why dry holes and poor wells occur in fields and why offset wells sometimes are totally different from and produce fewer hydrocarbons than the initial wells under apparently similar lithologic characteristics.

Later, when waterflood or gas injection systems were developed, it was hard or impossible to figure out why the water or gas tended to channel and flow in unexpected directions. One logical and simple explanation is the presence of differently shaped and low relief inconspicuous anticlinal structures instead of an apparent single structure. If this group of different anticlines and synclines is identified with the fracture patterns, interpretation is possible as individualized structures, probably with the same formational characteristics but with different pressure flows, gas instead of oil production, and different rates of water production than adjacent wells.

The concealed anticlinal structures and fracture patterns have influenced and controlled in many mature or younger fields the accumulation and distribution of hydrocarbons, regardless of age or lithology.

Examples of fractured reservoirs exist in every field where hydrocarbons are exploited. The simple explanation is that the whole earth's crust has been fractured, and it will continue to fracture by local and regional stress, fault movements, earthquakes, sun-moon-earth gravitational forces, mass settlement desiccation, and extraction of formational fluids. In general, anybody that drilled a well has been associated with fracture effects regardless if they were aware of it or not.

The least conspicuous fractured reservoirs are found in the Tertiary sands of the Gulf Coast, where best production is associated with faulted structures, and reservoirs often have high porosity and permeability. In most of these situations it is assumed that fractures are not an important factor in the production.

Studies have found that in many structures, if the oil reservoirs are there and regardless of the depositional origin, they are going to have better wells and a reduced number of dry holes if the wells are drilled on fracture intersections.

One of the best examples of how fractures control production in an excellent field is observed in the Devonian novaculties at Rojo Caballos field in Pecos County, Tex. Here the fractures are intimately associated with the faults and represent the entire porosity, permeability, and trapping mechanism of this field. Gulf Oil Corp. 1-D Zauk flowed 147 MMcfd of gas.

Another outstanding of example of 100% fracture porosity-permeability is East Puerto Chiquito Mancos oil field in Rio Arriba County, N.M., where 21 wells have produced more than 4 million bbl of oil from a depth of 1,900 ft.

In West Puerto Chiquito Mancos field, three wells, the 1, 11, and 13 Canada Ojitos units have produced more than 5 million bbl of oil and 2.7 bcf, with two of them still producing large amounts of oil and gas from fractured shale at 7,000 ft.

Sometimes fracture patterns represent or enhance only the permeability because primary porosity is present along with fracturing. Examples are found in reefs, mound reefs, mud mounds, and domes originated by diapiric evaporates or by igneous intrusions or extrusion, such as "serpentine plugs."

There is still another function of fractures that controls the presence of hydrocarbons in a producing field. When several vertical, well-developed fractures are filled or mineralized with carbonate or siliceous material, they could easily function as a permeability barrier if production is fracture-controlled. The production could be excellent within the "fracture enclosure" and poor beyond it, or vice versa. Similar application could explain why poor wells or dry holes are found in producing Gulf Coast fields.

The vertical and areal extension of fractures is a main concern of explorationists. Two different types of fractures have been recognized in relation to their areal extensions (Fig. 2). The first type, regional fractures (long fractures), can be observed for many miles. Short fractures, the other type, are abundant in most areas, and their length is less than 2 miles. We are aware that in many cases hydrocarbons have migrated long distances from their generating source. Consequently we prefer to map the regional fractures. However, short fractures could be mapped when detail is necessary for certain projects.

The vertical extension of long regional fractures is perpendicular to the bedding. That type of fractures has been observed going through horizontal Paleozoic sediments into an angular unconformity of Precambrian sediments (Fig. 3).

Naturally, other fractures will form at less than 900; some authors call them shear or tectonic fractures. In the field, it is difficult to identify which ones are regional or tectonic fractures, and this is the reason why wells are recommended on intersections of two or more fractures.

A drillable location could be chosen on a fracture trace as a last resort, knowing that if the fracture is of a shear type, the vertical trace is inclined, the deeper we drill, the further the borehole is going to be, at depth, from the fracture zone.

Many years of field geology was the main factor for making easier the geomorphic structural interpretations and the capability to identify fractures, lineaments, faults, and other geologic features from satellite (Landsat and SPOT), radar imagery, and stereo pair photographs. All this information can be mapped in detail on USGS 7'30" topographic quadrangle formats scale 1 in. = 2000 ft. The geomorphic structural regional interpretations have been mapped on planimetric maps scale 1 in. = 8,333 ft, also from USGS formats.1

Overseas, the industry has covered thousands of square miles in northern Mexico, where several gas fields were established. In South Africa a large area was covered for a helium pilot project. The southern portion of Belize was mapped with different techniques, including radar imagery, aerial stereo pair photographs, soil and vegetation maps, geochemical analysis, and strategic seismic lines.

In the U.S. the industry has mapped most of the oil producing states, as well as Washington state, where no production has been established.

In all of these areas of different geologic settings and geomorphic expressions, it was possible to identify fracture patterns and geomorphic structures of various shapes and attitudes. In some places like Texas and Louisiana fractures cannot be identified because of thick blankets of eolian sands and heavy timber that mask large surfaces.

The depths of wells drilled in areas influenced by fracture interpretation range from 600 ft to 18,000 ft. From these wells, it is possible to conclude that the producing fields were due primarily to the presence of fractures associated with the positive geomorphic structures. Other wells drilled on the same structures and away from fractured zones were either dry holes or very poor producers.

Another critical detail in the fracture concept is the width of the fracture zone (hereafter, referred to as the fracture). In desert areas on flat surfaces fracture expressions are about 30 ft wide. Transferring this width to the aerial photograph gives a fracture trace represented by a thin pen line.

It is necessary to differentiate between an individual fracture trace of about 30 ft wide, from the lateral width of a producible fractured area, where production could be established within 50-300 ft of a fracture trace. Wells are not recommended on a fracture trace much less 50-300 ft from the fracture. The optimum recommended location is on an intersection of two or more fractures. One well was staked between two producers 300 ft apart after it was estimated that a strong sand frac could reach either reservoir that the author corroborated to be on the same structure. We drilled through the sand and it was dry.

Many examples similar to this have been drilled where fractures make the difference between a dry hole or poor producer and an excellent well. It must be determined where the fracture intersections and geomorphic structures are located before a well is drilled. It is too late to start looking for fractures after drilling.

When fractures make the difference, it is known that lateral extension is critical. This distance becomes even more important when a fracture intersection point is transferred from an enlarged photograph to a topographic map and then to an ownership map, where the location is measured from lease lines. The best way is to stake the location in the field first, using the stereo pair photographs, and then survey the location.

Fractured reservoirs have been drilled successfully since the beginning. However, few geologists or engineers are aware of this fact, and few have understood, researched, or documented such reservoirs. If any production is established, the possibility of the presence of fractures is usually overlooked, and everything recorded is justified instead by assumptions of primary porosity and permeability. Today it is more apparent that the most productive fields are highly fractured or have been strongly influenced by fractures.

FRACTURE IDENTIFICATION

Presently, there are a variety of disciplines, methods, or tools for identifying fractures before drilling a well. The most common are: seismology, geochemistry, and radiometrics; and as remote sensing tools, satellite imagery, radar imagery, and stereo pair photographs.

Seismology: It is the most expensive tool. It works only if the fractures are inclined (i.e. faults) and have displacement that could readily be observed on seismic lines. A group of vertical fractures could be identified as a "noise zone" on certain seismic horizons. Areal extension of fractures is very difficult and expensive to determine by seismology, and it is almost impossible to detect fracture intersections and to evaluate their horizontal magnitude. Seismology was not designed for that purpose.

Geochemistry: This is a well accepted scientific and economical tool for detecting hydrocarbon signatures that came up from considerable depths through fractures and left enough evidence in the soil, near the surface. There are different techniques in the analysis of gas in the soil.

In any given area, the author strongly suggests, first, to construct a geomorphic structural map with the fracture identification on it and select areas where favorable structures have a larger number of fracture intersections; then we delineate such area for geochemical analysis.

Radiometrics: This surface gamma ray detection machine may work when it crosses a fracture. The principle is simple: it reads only radioactive material in the soil or outcrop, similar to what a gamma ray log reads in a borehole. There are 20 or more valid explanations for the machine to read high and low radioactive signatures. The fact is that every gamma ray instrument owner claims different reasons.

Magnetotellurics (MT), transient electromagnetics (TEM), microgravimetry, and micromagnetics: They are tools that give valuable information in hydrocarbon exploration and should be run after the area of interest has been detailed by geomorphic interpretation and fracture analysis mapping.

Remote sensing: Making photographic surveys from the air, scientists can do in 1-2 hr what might otherwise take 10-20 days.

Satellite imagery and high altitude photographs are unique tools for interpreting geomorphic structures and identifying fracture patterns that represent deep-seating structures and zones of greater permeability, which contribute to the generation and development of more, faster, and better drillable prospects. From this we selected stereo pair high altitude photographs for much greater detail. For example, on a typical evaluation of 1 00 sq miles, we first used Landsat at a scale of 1:100,000 and we could identify 16 regional fracture traces going through the specific area. With Landsat at 1:50,000 we could see 33 long fractures. Using the stereo pair photographs, we identified more than 300 fractures. The two Landsat images gave us regional fractures, some more than 30 miles long, while the aerial photographs gave us the excellent resolution we needed for the local fracture mapping for this project.

Landsat and SPOT satellite images can be used in petroleum geology as a large scale reconnaissance tool; also regional structural features can be mapped and the multispectral color gives information sometimes different than the black and white images. The outlook used SPOT special services. They merged panchromatic (B&W) 10 m resolution with multispectral (color) 20 m resolution, including geocoding at a scale of 1:24,000 or 1 in. = 2,000 ft. This process gave us the optimum resolution needed for detailed interpretations and excellent presentation and ground control for the USGS 7'30" quadrangles.

Stereo pair photographs are taken at different altitudes in black & white and infrared. In most of the projects, we recommend black & white and high altitude (NHAP) photos flown at 40,000 ft. These are the best stereo pair photographs for giving optimum detail in our identification and interpretation of geomorphological features.

In special desert areas of very low relief and poor drainage patterns, we recommend the use of stereo pair infrared photographs at a scale of 1:58,000 to identify fractures and faults, where the infrared would detect the higher heat of humidity preserved in concealed fractures and drainage representing the limits of geomorphic structures.

The optimum use and logical order of application for remote sensing techniques is as follows:

SPOT, merging panchromatic and multispectral images enlarged at scales of 1:24,000.
Black & white, high altitude stereo pair photographs enlarged to 1:24,000. In special areas color infrared high altitude photographs are recommended. Black & white and infrared photos are taken at the same time and altitude but with different focal length cameras.
For very small areas, with needs of greater detail, low altitude photos should also be used.
Overlays from photos and images completed with all geomorphic features can be overlapped to select, combine, and conclude the main structural findings. One final overlay can be made from them.
Field geology or corroboration in the field of all vital data should be done.
With the simple procedure of placing the overlays on a base map, it is possible to establish a relationship between the dry holes, marginal wells, and excellent producing wells, and all of the geomorphic structural features of the overlays. This also enables selection of prime leases.

The maps that result from this procedure can be the basis for selecting further studies on this and adjacent areas, including sub-surface data, recommendations of locations, where to run geochemical, micromagnetics, electromagnetics, or seismological surveys.

FRACTURED RESERVOIR RESERVES

Reserves vary from well to well in fractured reservoirs and are often difficult to determine due to lack of reliable fracture data. To estimate reserves, petroleum engineers require several core samples to determine size, spacing, and density of fracture pattern. Also required is the extension of the fractured area, keeping in mind that fractures have a longitudinal, lateral, and vertical network with potentially tremendous permeability. Engineers try to gather all this information after several wells have been drilled. However, initially, the potential reserves for a fractured reservoir can be tentatively assigned by analogy with the production history of wells drilled in nearby fields that have produced from similar fracture patterns, on the same structure of similar geomorphic structures. Large discrepancies are possible due to the individualistic properties of every fracture intersection.

MAPPING TECHNIQUE

In the last several years the interpretation of drillable structures on 7'30" topographic quadrangles has been refined. From this topographic map, another map can be constructed on clear on which to identify and highlight the topographic lows and highs, delineating with great detail the structures that these represent. On the same map, the fracture lines are transferred from aerial photographs or satellite images. The recommended fracture intersections, marked by a circle, represent the best permeable zone and sites to collect samples for geochemical soil-gas analysis. Ultimately, one of these intersections with the best structural position and geochemical signatures will determine the possible drillable locations.

If additional information is needed, one can select on the possible geomorphic structural map, the direction of planned seismic lines, or any other survey.

CLASSIC EXAMPLES

The information compiled on these maps was useful in locating producing wells in two different areas in northern Mexico, the Sabinas and Maverick basins, where gas was discovered between 7,000 ft and 10,000 ft in lower Cretaceous beds.

In Pecos County, Tex., the fracture concept was used to discover and develop fracture production from Devonian novaculite at 18,000 ft in Rojo Caballos field (Fig. 4).
In North Central Texas many shallow wells were drilled based on the fracture and geomorphic structures in upper Pennsylvanian sands at 1,000 ft.
In Maverick County, Tex., several wells were successfully drilled based on fracture maps at 7,000 ft. High pressure gas is found in members of the Pearsall and Upper Sligo formations. The Pearsall (James Lime) production is found exclusively within a 10 mile diameter geomorphic structure representing a James Lime wedge.
In the same county, excellent production has been found in the Chittim Arch at depths of 2,000-3,000 ft in San Miguel sands, Anacacho, Buda limestones, Eagle Ford sandstone, Georgetown, and Lower Glen Rose limestones. All the production has been found on geomorphic structures, with abundant fracture patterns.
In Fort Stockton, Tex., geomorphic interpretation was used to drill wells successfully in the Permian Queen sands.
In the East Texas basin, two planimetric quadrangles at a scale of 1:100,000 (Palestine and Crockett) were put together. The regional geomorphic structures are classic examples of how a geomorphic structural map should look. From that map were selected four 7'30" topographic quadrangles (Eunice, Halls Bluff, Lake Leon, and Stamire Lake) between Leon and Houston counties, Tex. The interest of these maps is their location in the Alabama Ferry field. The author did the fracture identification and the detailed geomorphic structure interpretation. By previous experience, it is known that the excellent production of lower Cretaceous, especially Glen Rose, Rodessa, Pearsall, and Sligo formations, is fracture controlled, and the low relief structures are critical, too.
Northeast of Odessa, Tex., a pilot area was mapped to analyze collapsed sinkholes where production is found. These sinkholes have excellent geomorphic expressions and are abundant in the Midland basin. The author is establishing a relationship between the size of the sinkholes, production, and the fracture patterns identified through high altitude infrared stereo pair photographs. Similar geomorphic features are being analyzed in Maverick County, Tex.
In McMullen County, Tex., a large area was covered with geomorphic structural interpretations. It was found that all deep and shallow production around the town of Tilden conformed with the geomorphic structures mapped.
In Grant, Rogers, Washita, Kiowa, Latimer, and LeFlore counties, Okla., the geomorphic structural maps and/or the fracture interpretations were the main tools for different operators to justify drilling. Commercial production was found in most of the wells, and the fracture influence varied according to reservoir type, fracture patterns, and location of leases on the structures.
West of Ashland in Clark County, Kan., a fracture and geomorphic structural map was used to develop and acquire additional acreage around producing fields.
In the east flank of the San Juan basin, Rio Arriba County, N.M., more than 160,000 acres were mapped around, three wells that have produced more than 5 million bbl of oil and close to 3 bcf of gas at 7,000 ft. In the same area is another producing field also in fractured Cretaceous Niobrara where 21 wells have produced more than 4 million bbl of oil at 1,950 ft. About 11,000 open acres with excellent fracture patterns were selected. The author strongly recommends a soil-gas geochemical analysis on the fracture intersections in order to determine the best surface hydrocarbon signature.
In Northwest Colorado, several fields were mapped with their fracture patterns in order to do additional drilling in the fractured Niobrara formation in Moffat County.
At East Elk Hills in Kern County, Calif., an oil field was mapped. Abundant fracture patterns were identified in this particular field located in the center of a geomorphic structure. Different geomorphic expression was found in Willows-Beehive Bend in Glenn County, Calif., where the terrain is practically flat. Identification of fracture patterns was accomplished.
North of Yakima, Wash., 200,000 acres were mapped on Umtanum Ridge. This area could represent one of the last large gas fields in the U.S. Fractures and faults were identified and, geomorphically, the folds and dips of the basalt could be interpreted to reflect the paleotopography sediments to 6,000 ft below the surface. It was assumed that the sediments and thick basalt cover are conformable and were folded to the present form during the Laramide revolution. Wells are going to be drilled based on our interpretation after a geochemical survey is run.
In Marion County, Ill., an area was mapped that covers the locations of Tonti and Patija oil fields. The production has been established from Silurian pinnacle reefs, the locations of which have reflected certain signatures on the surface where we can interpret on our geomorphic structural maps. Reef and shallower production has been enhanced or controlled by the fracture patterns. These reefs maintain certain trends that we can follow with geomorphic mapping. Where the author selects possible reef locations, he would recommend a geochemical and/or seismological survey.
West of Franklintown, La., a large area was covered with geomorphic structural maps. The surface expressions were good enough to determine the locations of shallow structures.
North of Bowling Green, Ky., fracture identification and geomorphic structural maps were successfully used in drilling several shallow wells. The modest production of the area is due to fracture controlled reservoirs that most of the operators have drilled at random or by off setting decent producing wells, eventually ending with abundant dry holes or non-commercial wells.

RECOMMENDATIONS

If you are in a position to start a prospect, the author would recommend a Regional Geomorphic Structural Map, scale 1:100,000. The author constructs these maps from USGS planimetric 30'x 60' quadrangles and interprets the structures with their axes in areas known to be appropriate for the technique. On these economical maps, an operator can select possible structures, and the code names of the 7'30" quadrangles. These 7'30" topographic maps are the base for our Detailed Geomorphic Structural Interpretation, scale 1:24,000 or 1 in. 2,000 ft. If one splices together four of these maps, an overlay results of a 15' x 15' quadrangle equivalent to about 160,000 acres or 236 sq miles.

Now, one is in an excellent position to select subsurface or any other kind of available data. He can choose acreage and wait for a fracture interpretation before leasing it. If additional information is needed, the author can suggest the next survey and its direction and density. It is also recommended to verify most of the mapped interpretation in the field and locate the drillable sites on the terrain from the stereo pair photos.

The author can assist the petroleum engineer or drilling contractor with preventive recommendations for the drilling fluid. High concentrations of bentonite or any amount of barite are not recommended. Other drilling fluids do not permanently close or seal low permeability or fracture reservoirs. Other small details need to be observed when logging, perforating, treating, and producing such reservoirs.

The present trend of horizontal drilling needs to focus more intensively in mastering or at least understanding the fracture concept, mainly their patterns, width, length and their orientation, before the well spot is decided and the length and direction of the horizontal borehole are determined.

Every operator needs to have a fracture interpretation and geomorphic structural map to decide the best location and direction in order to cross the larger number of fracture intersections and decide whether it is more convenient to go downdip, updip, or along strike.

Some dry holes or poor producers were drilled on or near a fracture, and their horizontal orientation went away from the fracture. This fact indicates that one cannot assume that any horizontal well is going to cross or intersect, 11 producing fractures." The experience obtained in drilling vertical wells on regional and local fractures has indicated the importance of mapping only regional traces, consequently an operator must be selective in choosing the best fracture patterns. In this sophisticated drilling project, the author strongly recommends running a soil gas geochemical analysis from fracture intersections and along fractures.

Keep in mind that not all geomorphic structures represent deep-seated structures with commercial oil deposits, and neither do all fracture intersections carry hydrocarbons. One of the most important factors in this industry is the necessity of using different tools or techniques in order to reduce the risk of drilling noncommercial wells.