The integrated radiation environment at well sites-an adjunct to petroleum exploration

Frederic R. Siegel
George Washington University
Washington, D.C.

Reuven Chen
Tel Aviv University
Tel-Aviv, Ramat-Aviv, Israel

J. Eduardo Vaz
Venezuelan Institute of Scientific Research
Caracas

V.K. Mathur
U.S. Naval Surface Warfare Center
Carderock, Md.

Integrated thermoluminescence (TL) radiometrics was used as an exploration adjunct to locate oil well spud-in sites at Helez field, Israel.

The field combines stratigraphic-structural traps on a NE-SW tending faulted anticline (Late Cretaceous-Early Tertiary) tilted gently to the east and downfaulted to the west. Neogene transverse faults divided the field into different fault blocks with oil trapped in numerous small, separated reservoirs. Many fault segments have not been drained by existing wells. The targeting of spud-in sites to drain these segments is assessed in this study.

Sixty-seven sites were studied in a 14 sq km area. These included wells that were or still are oil producers. Radioactivity signal accumulated during 100 days with buried TL dosimeters ranged from 72-201 nanocoulombs (nc). Twelve of 17 oil producer sites had low radiometric values. With drilling done sequentially from lowest to higher radiometric values, the first six wells, with radiometric values ranging from 97-116 nc, would have been completed as oil producers.

Introduction

Several surface and near-surface measurements have been used in oil exploration.^1-10 These include concentrations of natural gas components (e.g., C₃H₈), chemical elements (e.g., I), radioactivity (e.g., from Rn), and microbes (e.g. Mycobacterium rubrum var. propanicum).

The measurements focus on targeting small areas in large prospects that have a greater probability of containing oil and natural gas. In this way exploration front-end costs can be lowered because geophysical surveys can be limited to well-defined areas.

Surface work premise

The application of these methodologies in oil and natural gas exploration is based on the premise that vertical migration of gases from hydrocarbon traps to the surface develops a spatial relationship between surface or near-surface measurements and the traps. The measurements may be of a product that originates from an oil pool, such as C₃H₈ or radioactivity, an alteration product from the interaction of vertically migrating gases with surface or near-surface earth materials, such as "abnormal" carbonates designated as DC, or a parameter that develops as a result of reaction of the vertically migrating gas with the ecosystem, such as propane gas that is nutrient for Mycobacterium rubrum var. propanicum and stimulates its own growth.

We report here on a radioactivity measurement. The hypothesis is that uranium in formational waters is precipitated when it enters the reducing environment of an oil trap. Further, there is vertical migration of daughter products from the decay of uranium, especially radon, through the rock column overlying an oil trap. The isotopes ²²²Rn, ²²⁰Rn, and ²¹⁹Rn are generated midway through the decay chain of ²³⁸U, ²³²Th, and ²³⁵U. ²²²Rn, with a 3.8-day half-life, can migrate vertically through the sediment or be transported by water or natural gas. In theory, if an anticlinal structure is being evaluated, low radiometric values can be expected where the trap is sealed by secondary deposits (carbonate), sulfate, sulfur) which are not as permeable to gas escape compared to lateral areas. This gives a "halo" or partial "halo" distribution of higher value measurements away from a trap where the seal is missing or breached by strain fractures.

Stratigraphic or fault traps give a linear anomaly pattern that locates the edge of a trap. Offset anomalies can result from fault or fracture systems in the strata overlying a hydrocarbon trap so that knowing the geology is essential to interpret surface or near-surface radioactivity fields.

The basic problem: a logical solution

There has been some success in surface and near-surface studies of areas with known oil and gas production.^{3 11 12} However, this has not increased interest in the application of radiometric technologies which were not systematically tested and not always clearly understood by geologists or scientific managers.

Climatological conditions and measurement times can affect the reproducibility of radiometric data. The rate of gas flow is influenced by factors such as barometric pressure, differences in temperature between surface and near-surface environments and the air, time of day of sampling, wind and/or precipitation conditions, relative humidity, depth to water table, soil moisture, and the frozen or thawed state of a soil.^13-16

Transient measurements made with electronic field equipment during short-term periods give results that may be inconsistent, nonrepeatable, and even wrong with respect to known oil pools. Thus an integrated signal is preferred.

We have used thermoluminescence dosimeters (TLDs), buried for 3-4 months, to obtain integrated radiometric data at a producing oil pool in eastern China¹⁷ and at a major prospect in the Tarim basin, western China.¹⁸ TLDs are solid state materials in which the total quantity of light emitted and measured as the material is heated is linearly proportional to the absorbed radiation dose.¹⁹

In the two reports on China, the aim was to determine if oil exploration areas could be reduced to favorable smaller areas for seismic surveys. They were successful.

At Shengping oil pool, about 35% of the 112 sq km study area was targeted as propitious for containing oil and/or natural gas. Dry holes were located outside the targeted areas as hypothesized.

Seven oil producing wells were located in the targeted areas. One of these was drilled after the TLD study was completed. Three of four wells with noncommercial shows of oil were in areas targeted by the TLD measurements.¹⁷

The Shengping field producing area was increased by 6 sq km and production by 1 million tons/year of crude on the basis of combining TLD data with geological and geophysical data.²⁰

At the Takala prospect, the TLD data for an 80 sq km area were compared with Rn and DC data generated at the same time. The TLD data were more definitive in reflecting the faults system in the study area.

A similar relation between TLD and Rn data has been reported for a sand dune area in Inner Mongolia, China.²¹ For the wells drilled at this prospect, one oil producing well was on a TLD target, and one dry hole was away from a TLD target. Favorable areas for hydrocarbon exploration were indicated by TLD radiometrics.

Purpose of the study

This study evaluates integrated TL radiometrics as a method to complement geological and geophysical data to locate spud-in sites for step-out production in an existing oil field, Helez field in the southern coastal plain of Israel (Fig. 1) [7,386 bytes].

Sample sites were selected with this objective. This project was important because tectonic activity isolated oil in different fault blocks in numerous small separate reservoirs at Helez field. As a result, many fault segments in certain reservoirs have not been drained by existing wells.

It is estimated that 19 million bbl of oil remain to be extracted from these inadequately drained reservoirs.²² In addition to indicating spud-in sites for step-out drilling, the TLD data may suggest the possibility of opening new pay zones in Helez field by deepening two existing wells.

Geology
Helez formation

The Helez formation underlying the Israel southern coastal plain is composed of alternating shales, sandstones, sandy shales, limestones, and dolomites.^23-24 It reaches depths greater than 1,700 m, is up to 300 m thick, and becomes sandier to the east.

The sandstones constitute about 10% of the formation and are the main oil reservoirs. Most oil is found in the more porous sandstones that were deposited to the east in coastal environment tidal channels or lagoons. Intergranular porosity can reach 20-30% of the rock volume, and permeability reaches 2,000 md.

The pay zones in the Helez formation are 1-12 m thick and are mainly at depths of 1,590 m to more than 1,620 m with a deeper zone at about 1,675-85 m.²² Oil is also found in dolomitic rocks that comprise the upper section of a reef complex. There is smaller production to the west where the formation is more shaly. A marine sandstone facies there has low porosity and contains little oil.

There is no oil in the older Early Cretaceous Gevar'am formation, a thick series of impervious dark to black shales.^{22 25} The Helez formation is conformable and interfingers with the Gevar'am formation.

Where the Helez formation pinches out in the shales, oil traps have formed (Fig. 2 [28,847 bytes]). The Gevar'am shales are the seals for the traps.^{25 26} Oil is also found in fractures where Jurassic limestones unconformably underlay the Helez formation.

Helez structure, traps

The oil field is located on a marked Cretaceous to Eocene faulted anticline that coincides with a depositional hinge belt.²⁵ The structure is tilted with a gentle dip to the east and is downfaulted to the west (Fig. 3 [22,385 bytes]).²²

Folding is related to deep-seated compressional faults. The structure has a N-S Jurassic axial trend in the northern part and a Late Cretaceous-Early Tertiary NE-SW trend in the southern part. Faults in the area have been related to uplifting and tilting at the end of the Jurassic, an Alpine folding phase during Late Cretaceous-Early Tertiary times, and Neogene tensional movement.²⁷

Helez field is a combination stratigraphic-structural trap on the NE-SW trend. Thinning and shoaling of Helez formation sands occurred updip (to the west) leading to the formation of stratigraphic pinchout traps sealed by overlying and interbedded thick shales (Fig. 2). Although the porous sandstones of the Helez formation interfinger with the Gevar'am shales beds in a landward transition zone (to the southeast), they may pinch out before they reach the shales.²⁶ In this case, the sandstones are separated from the shales by a belt of tight oolitic sandy limestone alternating with sandy shale acting as oil trap seals. There is a dolomitic reef complex as well.²³

Oil migrated and was trapped during the Neogene after the major tectonic movements. Migration was facilitated by post-folding tensional transverse faults and fractures that cross Helez field ascending from deep layers in the western basin.

On the basis of the close association and interfingering of the Gevar'am black shales with the Helez formation oil bearing sandstones, it was proposed that the shales were the source rocks.^{23 24} However, data on biomarkers and stable isotopes in the Gevar-am shales and the Jurassic Barnea limestone formation indicated that Barnea formation samples represent both the rock type from which the oil was derived and the migration path.²⁵

Subsequently, transform adjustment faults divided the Helez structure into blocks that separate the Lower Cretaceous pay zones into different reservoirs.²²

Methods

LiF (Ti,Mg) TLDs (3.2 x 3.2 x 0.9 mm) were prepared for field burial following and manufacturer's recommendations. These were kept in a light-tight bag for transport to and from the field area.

Sixty seven sites representing 21 oil producers, six dry holes, and 40 undrilled locations were chosen for TLD measurements in a 14 sq km area. Dosimeters were put in holes at 50 cm depth and covered with excavated material. After 3-4 months, the dosimeters were recovered at 58 locations.

The accumulated radioactivity dosage of the 200° C. TL glow-peak was measured using a Harshaw Thermoluminescence Dosimetry Reader. Radiometric readings were normalized to 100 days to be able to compare readings between measurement sites. More than one dosimeter was measured for each site in order to assess the precision of the radiometric data.

Further statistical evaluation of the data was not done for two reasons. First, the study area is an existing oil field and is expected to have low radiometric values generally consistent with oil producing areas. Second, the sampling was not random but intended to evaluate measurements from known oil producer wells, known dry holes, and related areas.

Results, discussion
Integrated TLD values

Normalized accumulated dosages ranged from 72 to 201 nc. The mean accumulated dosage was 135 nc. Precision of replicate samples was 3.4%.

Radiometric measurements were designated as low values if they were < 130 nc.

Fig. 5 [30,802 bytes] presents a contour map for the radiometric values. The value of 100 nc to the right of center on Fig. 5 is the location of the discovery well shown on the Helez field anticlinal structure (Fig. 4 [38,712 bytes]). Values are high in zones to the east, north, and northwest of the structure.

Dosimeters were recovered from 17 of the 21 producer well sites where they had been buried. Dosimeters were not buried at four producer wells. Samples were not recovered at five undrilled sites.

Table 1 [32,225 bytes] lists the low integrated radiometric values (< 130 nc) in sequence and their relations to producer oil wells, undrilled sites, and dry holes. of the 25 locations represented, 12 correspond to producers, 10 to undrilled sites, and three to dry holes.

The lowest 14 readings (72-116 nc) are at six producers and eight undrilled locations. The next four values are paired (two with 118 nc and two with 120 nc), and each pair represents a producer well and a dry hole. Of the last seven locations (121-128 nc), four are at producer oil wells, one (123 nc) is at a dry hole, and two are undrilled.

Seven of the 12 oil producers listed in Table 1 are on the principal anticlinal structure between the faults to the NE and SW (Fig. 5). This observation reinforces the concept that at an anticlinal structure, albeit with stratigraphic traps, low radiometric values can be expected at the surface spatially linked with oil traps where they are sealed by secondary cap deposits or other poorly permeable rocks.

High values (142, 159, 163, and 172 nc) to the east and northof the anticline are at four oil producer wells (Fig. 4). This suggests that trap seals have thinned away from the crest of the structure and/or have been breached, perhaps from strain fractures. Here, greater concentrations of gas carrying the radiometric signal have probably migrated vertically giving the higher radiometric readings. The locations of these producer wells would not have been recommended for further study on the basis of integrated radiometrics alone.

Six of eight dry holes in the study area were also evaluated (Fig. 4). One has a high value (197 nc), two are below the mean (132 and 130 nc), and three are included in the group (118, 120, 123 nc) that was used to designate targets (Fig. 5).

The dry holes with values of 118 and 120 nc are in the southwest corner of the study area. It is possible that they were not drilled deep enough to penetrate the Helez pay zones28 and hence the underlying Jurassic rocks.

The oil well immediately to the east of the 118 nc and southeast of the 120 nc sites has a value of 110 nc (Fig. 5) and produced from a Jurassic limestone. The 118 and 120 nc dry hole locations merit consideration for deeper drilling.

If the low to progressively higher radiometric values were used in the selection of sites for the oil producer wells, as a complement to favorable geological and geophysical results, the first 12 wells drilled would have corresponded with 10 oil producers and the two dry holes at 118 and 120 nc cited above (Table 1, Fig. 5).

From the results, the undrilled low value sites given in Table 1 become priorities in evaluation for step-out drilling. The selection of any of these sites for drilling should be based on a specific location analysis of geological and geophysical data.

Two targets that are favorable for step-out drilling have values of 72 and 108 nc (Fig. 5). If one or both of these targets are drilled and show oil, the concept of using integrated radiometrics for locating spud-in sites for step-out production or exploration wells in an undrilled area would be supported.

Problems, solutions

At present, no commercial laboratories routinely do analyses for TLD based projects. However, the equipment is commercially available.

Dosimeters are reusable many times if cleaned and handled properly. Once recovered and brought to the laboratory, more than 100 chips can be processed in an 8 hr work day. Thus, doing a study with 100 sample sites, using two dosimeters per container and two containers at a site to check precision would require 400 TLD chips. In this situation, after chip recovery radiometric data can be generated in 4 days or less.

The burial time of 3-4 months is a problem for oil industry managers who view this as too long a time lag in their exploration programs. This problem was addressed with the recent development by Chinese scientists of LiF (Mg,Cu,P) TL chips that are much more sensitive than the same mass of TF (Ti,Mg) chips that are being used.²⁹

Also, the high signal to noise ratio reported for these chips suggests a significant reduction in uncertainty at low dose measurements compared to the traditional LiF (Ti,Mg) dosimeters. Chinese geochemists have had success using the LiF (Mg,Cu,P) dosimeters at oil fields in varying land-use (e.g., agriculture, grasslands, sand dunes, urban) and climatological environments.^{20 28 29}

Burial time for integrated radiometric measurements with the more sensitive chips can be reduced to one month. Recent research suggests that dosimeter burial time can be reduced to 10 days.²⁰ Also, measurements have been made on site with a portable thermoluminescence dosimetry reader. With computer coupling, real-time maps can be generated and updated daily to provide guidance to geological and geophysical work in oil and gas exploration programs.

Integrated measurements can also be made during times of adverse field conditions. For example, in tropical rain forests, dosimeters can be buried before a rainy season begins and collected months later when field conditions are favorable.³² Similarly, in temperate or peri-glacial regions, dosimeters can be buried before the onset of winter, register data during the time when field work is impractical or impossible because of cold or snow cover, and be recovered under good field conditions. The superiority of buried LiF (Ti,Mg) dosimeters in measuring the natural radiation in the Antarctic versus on-site scintillometer or radon track-etch measurements has been demonstrated.³³

In addition, there is little impact on the environments where TLD measurements are made. Plugs of soil are excavated, dosimeters planted, and soil replaced. For dosimeter recovery, the same plug removal-plug replacement procedure is done.

As the search for oil and gas expands into environmentally sensitive areas such as the Arctic National Wildlife Refuge, the use of TL data can define small favorable areas to be studied using seismic analysis. This limits the number of trails necessary for seismic surveying.

Conclusions

Radiometric measurements made using buried LiF (Ti,Mg) TLDs identified 12 of 17 oil producer wells as low activity radiometric targets. This supports the hypothesis that vertical migration or radioactive isotopes spatially links subsurface oil deposits to near-surface measurements.

Undrilled locations with low radiometric readings should be evaluated with geological and geophysical data. If they are drilled and find undrained reservoirs, the case for using radiometrics to locate spud-in sites for step-out production is furthered. Integrated TL radiometrics used with geology and geophysics could be an additional useful strategy in oil and gas exploration.

Acknowledgments

The authors thank Lapidoth, Israel Oil Prospectors Ltd., for assistance that allowed us to carry out this study.

References available from first author.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.