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Integrated Analysis of phytogeochemical anomalies and oil Remote Sensing abnormalities for prospecting oil pools in arid region in the west of China

Yang Hong,Zhang Jianzhong,Zhu Boqin,Li Jiahong
Institute of Remote Sensing Applications, CAS P.O. Box. 775, Beijing 100101, P.R. China


Abstract
Compared geobotanical and phytogeochemical anomalies in Xiquan test site with those in the known T22 oil field, we found that the characteristics were similar in these two areas: (1) Plants of the two areas has obvious pathological changes: dominant Plant Suaeda physophora is perennial, dense but growing poorly. A lot of individuals had dead leaves, and these were many oven patches of dead plants in the community, while Reaumuria soongorica has the similar fatal Main companion species Suaeda acuminate has a long term of growth, high density and clear phenomenon of turning to red in ‘Xiquan, Dominant plants of Artemisia boroculensis around T22 oil field were crowded and wick; Most of the major accompanying species Suaeds dendroides were dry and dead, while some stems and leaves of the alive changed to red. (2) phytogeochemical unusualness were stable and can be tested: ash content of T22 oil field was higher than that of compared area, and that of Xiquan was the highest of the three. Trace elements of the plant on T22 field were very rich, and P Ti, Fe, Al, Fe/Mn in Artemisia boroculensis were 1.3 – 7.7 times as much as those in contrasted area; The elements of Sr, Li, Ti in Sueda physophora and Reaumuria soongorica in Xiquan were also high.

Phytochemical anomalies were well tallied with oil Remote Sensing anoamalies. It has the characteristics of oil region in Xiquan that was surposed a prospecting oil area.

Introduction
The surface geochemical condition can be directly detected by using Remote Sensing in sparse vegetation area; In thick vegetation region. Vegetation has been screening and shielding surface geochemical features, so the influence of vegetation must be considered in Remote Sensing to oil exploration. Meantime, abnormal surface geochemistry must cause the anomalies of plant organization, plant physiology and population features. These anomalies are the keys to detecting oil resources.

General setting of Xiquan test site and T22 oil field
  1. Geographic setting
    Xiquan and T22 field lie in the north foot of Tianshan mountains. T22 well is 15 kilometers southeast of Xiquan. The relief is south-high and north-low. T22 well is located at the side overlapping part between two pluvial funs. Xiquan is situated at the alluvial plain. They are both in typically dry and semidry continental climate zone. The average annual precipitation is 170 to 160 mm, the annual temperature charges from -360 C to 400 C and the average annual evaporation is over 2000mm.

  2. Geologic Setting
    Xiquan test site is located at the Fukang sag (Figure 1). Oil generating strata of Fukang sag is large and thick enough to be a good source bed. Hydrocarbons matured lately and didn’t scale the height of ejecting hydrocarbon until Tertiary period do they escaped the two disasters of strong denude before Cretaceous period and Tertiary period. Unconformity trap, became a perfect stratigraphic trap. T22 oil well lies on the Xiaoquangou anticline which is formed in Jurassic and between Fukang faulted zone and Ganhezi faulted zone. T22 is a paying well.


    Figure 1. Structure of Xiquan test site and T22 field

  3. Soil and Vegetation
    There grows desert calc soil and cal-alkaline soil in Xiquan but grit and desert gravel soil in T22 region. Both of the two above regions are chiefly covered by desert vegetation. The dominant species are extra rid under - shrub and half – shrub, annual and perennial herbs, spring ephemeral plants and ephemeral like plants. The communities were divided as follows according to the habitat of dominant plants.
a. Xiquan:
  • Reaumuia soongorica community;
  • Haloxylon ammondendron community ;
  • Suaeda physophora community;
  • Achnatherum spelnden community;
  • Wildland community;
b. T22 Field
  • Nanophyton erinaceum community;
  • Artemisia boroculensis and Napophyton erinaceum community;
  • Nanophyton erinaceum and ceratocarpus arenarius community.
The correlative between phytogeochemical abnormally and oil Remote Sensing anomalies in Xiquan and T22 field. Eight work bands were selected to detect oil pool by using airborne shortwave infrared split spectral scanner. Their central bands are :1.600, 2.087, 2.143, 2.200, 2.250, 2.300, 2.330 and 2.450 mm. obtained short wave infrared Remote Sensing data were used in computer image processing and acquirement of oil Remote Sensing anomalous information. Supervised classification was conducted by regarding known oil pool as a discipline area. Oil Remote Sensing anomalous regions were demarcated by integrated analyzing geologic structures and oil Remote Sensing anomalous sketch map whose main component was soil hydrocarbon.

A. Oil Remote Sensing anomalies
whose shapes were pieces or spots were distributed into semiarc around T22 well. The anomalies were well conformed to the horse of anticline, also perfectly tallied with the oil-bearing areas (Figure 2).


Figrue 2 Composite map of oil Remote Sensing anomalies and the anticline in T22 oil field

Xiquan oil Remote Sensing anomalies assumed a typical semi-ring composed of complicated spots with large scope and strong information were tallied with a nose anticline (Figure 3). Nose anticline that lies on the east slope of Fukang oil-bearing sag is a prospecting oil bearing structure. Especially, B27 oil well that is in the east of the nose anticline had produced paying oil.


Figrue 3 Compoisite map of oil Remote Sensing anomalies and Jurassic Seismic back wave anomalies in Xiouan test site

B. Geobotanical and phytogeochemical abnormalities
  1. The plant communities and individuals grew unusually: (1) In the coen composed of the same species, which lied in a alike habitat, the individuals were tender and short but dense in T22 field. The individual number of dominant species Artemisia boroculensis was 2.7 times as much as much as that in the contrasted region while each individual cover was only 0.325 times. Reaumurai soongorica in Xiquan grew poorly and shortly. (2) Geobotany developed anomalously, and there existed normality and abnormality. In T22 oil field, most of Sueada dendroides individuals were dry and dead. The stems and leaves of the alive had turned to red. In Xiduan test site, many individuals of Suaeada physophora had dead stems, and there were oven patches of withered plants; Suaeda acuminate whose stems and leaves were red, bright red and purple grew densely as a background synusia. (3) There were many species and biotypes here.
  2. Ash content was terminated by plant success ional differentiation and environment. To the time species, ash composition of T22 field was higher than that of compared area. This was caused by the geotemperature of oil field was so high that plant roots observed more minerals and the amass was increasing, therefore ash competent was higher. Plant ash content of the same species in Xiquan was higher than that in T22 oil field.
  3. The content of plant mineral elements that possess important functions in plant physiologic action was complex. Plants are sensible in physiology to the trace element variance. A number of them selected specific trace elements of environment, which acted as a bio-accumulator, to increase abnormal differences. Excessive and lack trace elements will change the growth of plant or eve cause their death. The composition of P, Ti, Fe, Al, Fe/Mn in Artemisia boroculensis were 1.3 – 7.7 times as much as that in compared area. The difference was great. The content of some elements in the same species in different places was relatively steady. Dominant plants distributed over high hydrocarbon seeping bands had notable geochemical anomalies whose scopes were smaller than high hydrocarbon seeping area, which means phytogeochemcial abnormalities emerged certain unusual hydrocarbon conditions and could be the marks of reservoir. In Xiquan test site, a lot of plants accumulated trace elements of Sr, Li, Ti, Cd whose content were 15 to 130 times as much as the average value of continental plants while Zn and Cu were only one twelfth of the average.
  4. The correlativity of phytogeochemical anomalies and oil Remote Sensing anomalies.
The continent abnormalities of characteristics elements in dominant species Artemisia boroculensis in T22 field well accorded with oil Remote Sensing anomalies. The scope of withered and changed red Suaeda dendroides, the content anomalies of characteristics elements in Anabasis elarior and those in Artemisia boroculensis (Figure 4) overlapped each very well. The better coincidence was not accidental: first, that content anomalous peak velue was 300 m away north of T22 well demonstrated that well drilling hadn’t affected anomalies; second, that characteristic elemental anomalies were in keeping with oil Remote Sensing abnormalities showed that both of them were the reaction of hydrocarbon microseepage from reservoir. Overlapping the characteristic elemental anomalous integrated maps of Reaumuria soongorice, Suaeda acuminate and Suaeda physophora that dominated over Xiqusn extensively, we found that the south, southwest and northeast parts overlapped best, which were considered to be in the first group: the west and north part overlapped better, which were thought to be in the second group. The integrated phytogeochemical anomalies coincided basically with oil Remote Sensing abnormalities (Figure 5).


Figure 4. Correlative map of characterastic elemental content of Artemisia boroculensis(left) and Anabasis elatior(right) and oil Remote Sensing anomalies in T22 oil field

Discussion
  • From the comparation with T22 oil field, Xiquan test site had the characteristics of an oil field. Xiquan is a prospecting area.
  • From the tectonic analysis, Xiquan area also showed favorable oil prospecting.
  • From mechanical analysis, it is not accidental that the good coincidence between oil Remote Sensing anomalies and phytogeochemical anomalies. It can be surposed that this result is caused by the underground prospecting oil pool that changes the surface circumstances by hydrocarbon microseepage.
Conclusion
  • Phytogeochemical anomalies of some species above oil pool are relatively stable;
  • Plant communities, population features and plant growth are abnormal on oil reservoir;
  • Phytogeochemical abnormalities changed spectral features, which set off Remote Sensing unusual. They are positively correlative.
Integrated analysis of high correlativity of phytogeochemical anomalies and oil Remote Sensing anomalies with substructures in Xiquan test site showed that Xiquan was a prospecting oil area.

Acknowledgements
Professor Zhu Zhennai, Institutes of Remote Sensing application, CAS, provided thoughtful discussions, helpful suggestions and a useful review of the manuscript. Figures were prepared by Zhang Yili, Institute of Geography, CAS.


Figure 5. Correlative map of integrated phytogeochemical anomalies and oil Remote Sensing Anomalies in Xiquan test site

References
Abrams M.J. The Joint NASA / Geosat Test Case Project, AAPG Part 2, Vol. II, AAPG BOOKSTORE, 1984.

Barret, N.R. Remote Detection of Geobotanical Anomalies Associated with Hydrocarbon Microseepage, Proceeding 3rd, ISRSE, Clorado, pp183 195, 1983.

Feder, A.M., Remote Sensing detection of microseepages in hydrocarbon exploration, 4th Thematic Conference, Proceedings of Remote Sensing for Exploration Geolog