Selasa, 19 April 2011

VOLCANIC RESERVOIR CHARACTERIZATION OF JATIBARANG FORMATION BASED ON AN INTEGRATED STUDY OF PETROGRAPHY, CORE, FMI, AND WELL LOG



Presented by :
Zeindra Ernando (PT Chandra Bumi Sakti)
Achmad Fathoni (PT Pertamina EP Region Jawa, Cirebon)


ABSTRACT

Volcanic reservoir of Jatibarang Formation has been long object to study. The main reservoir Jatibarang Formation reservoir consist of naturally fractured reservoir. This volcanic reservoir divided into three facies: massive tuffacoeus sandstone, laminated tuffaceous sandstone, and conglomerate facies. Realistic characterization of volcanic reservoir is a problem because logging tools are not designed to detect the volcanic reservoir.

An integrated study of petrography, Scanning Electron Microscope (SEM), Side Wall Core (SWC), Full bore Formation Micro Imager (FMI), and well log becomes important thing to define characterization of volcanic Jatibarang Formation. Matrix density and porosity type are important parameters to get appropriate porosity value in each facies. Well test data are used to determine the resistivity water and it will be influenced by value of saturation water. Permeability model is got from Wyllie-Rose formula in each facies.

This paper describes the characterization of volcanic reservoir of Jatibarang Formation to get a model or parameter for each facies based on integrated well data. The well analysis must be done for each facies of volcanic reservoir. Lithology of volcanic Jatibarang reservoir is very complex. It presents difficult to compare relation between reservoir engineer and petrophysicist to get realistic parameter of formation. Matrix density, porosity, and permeability model are important parameters in characterization of volcanic reservoir, so it can be used to another un-cored wells among this well.


INTRODUCTION

The area study is located in Cipunegara Low, North West Java Basin. North West Java Basin is known as the hydrocarbon producing basin. The hydrocarbon is produced from Jatibarang volcanic, lower Cibulakan (Talang Akar and Equivalent Baturaja Formation), and upper Cibulakan, Parigi Formation. (Reminton and Pranyoto, 1985). Jatibarang volcanic have been date as Eocene to Oligocene. They were deposited in the newly developing back arc basin during the initiation of the present subduction zone south of Java (Hamilton, 1979 vide Thomas Kalan et al., 1994). Jatibarang Formation consists of unfossiliferous, varicolored and mottled tuffs, porphyry andesite, basalt and red claystone and also was deposited on top of Pretertiary basement troughs or grabens and low area. The barren sediment with their variegated color, most likely represent a continental to fluvial environment. (Arpandi and Padmosukismo, 1975).

Jatibarang Formation, North West Java Basin has been studied for long time. The main reservoirs of Jatibarang Formation consist of naturally fractured reservoir. Reservoir characterization of volcanic rocks in the “Ratu-01” well must be reviewed and the basis for a research topic in the context of preparing final project researchers because of logging tools are not designed to detect the volcanic character, so the author has to do the integrated study of well data. Based on integrated study of petrography, SEM, and sidewall core analysis from “Ratu-1” well, the Jatibarang Formation may be identified as altered volcanic sediment as tuffaceous sandstone deposits, (petrography sample depth 3000 – 3002 m), conglomerate as quartz granule conglomerate (petrography sample depth 3051 m) and sandy conglomerate (petrography sample depth 3062.5 m). This volcanic reservoir is divided into three facies: massive tuffaceous sandstone, laminated tuffaceous sandstone, conglomerate facies. These group facies are very important in the petrophysic calculation to get a reservoir model (texture, mineralogy and diagenetic process), so we would get relatively appropriate reservoir character withthe real situation.


METHODOLOGY

Due to the complexity of the volcanic reservoir, it was decided that geological work would have to be detail to allow an independent evaluation of the well. The complexity would be influenced by depositional texture, mineralogy and diagenetic processes. This complexity would influence porosity type and water saturation estimation. Therefore integrated study of petrography, SEM, and sidewall core are needed on Jatibarang Formation. It is also have to compare with FMI to identify porosity type. Realistic characterization of volcanic reservoir is a problem because logging tools are not designed to detect the volcanic reservoir. This integrated study aims to determine the characteristics of volcanic reservoir based on porosity type and mineralogy on “Ratu-1” well. It is planned to further develop the technique for the interpretation of other volcanic reservoir among area study.


FACIES ANALYSIS

The result of an integrated study of petrography performed on few SWC and cutting samples which supported by SEM and XRD analysis that are selected samples respectively from the “Ratu-1” well in Jatibarang Formation will be discuss in terms of their depositional textures, composition, diagenesis, and reservoir quality and their controls, as below.

Based on petrography analysis the samples, there were three main facies which would be analyzed in this research; massive tuffaceous sandstone, laminated tuffaceous sandstone, and conglomerate.

Tuffaceous Sandstone Facies

In general the tuffaceous sandstone (sample depth 3000-3002 m) of Jatibarang Formation commonly very fine grained to clay in sizes. Based on petrography data, this lithology type consists of monocrystalline quartz (3%), carbonaceous materials (14.00%), and altered product of argillaceous clay / sericite as ground mass matrix (81%). Relict of flowing structure of glass mass may able to detect (K-P, 4-6, Figure 2). In the other cuttings a concentrations of epidote mineral as a product of geothermal alteration was identified. The result of X-ray diffraction (XRD) analysis on cutting sample at depth 3000-3002 m demonstrates that the rock consists predominantly of quartz (71%, from devitrification process of glass), plagioclase (5%), pyrite (1%), illite (6%), kaolinite (10%), and chlorite (6%).

The diagenetic event occurs in this volcanic facies is very difficult to be detected except weathering product of alteration glass to clay minerals. Probably minor dissolution of selective glassy matrix is to form secondary porosity. Petrographic observation under the microscope and scanning electron microscope (SEM) examination show the rock in this facies has low porosity (2.00% by volume). Interconnectivity among pores is very poor and it is represented by local dissolution pore type. Based on petrographic examination on their texture, the depositional environment of this volcanic sediment probably is in an open sub-aerial of low land area.
Based on FMI, this facies is comprised of mainly volcanic tuff. Tuff are predominant massive, although locally remnant laminated facies are observed and in places demonstrate well developed scour and fill structures. Laminated tuffs are best interpreted as having been deposited in subaqueous conditions.
There is very little indication of stratigraphic zone in this facies. What is appear on the image to be a major erosive surface occur at 3032 m. Laminate tuffs are more common below this surface (major erosive surface), indicating more obvious waterlain sediments. Above this surface tuffs are generally more massive (major erosive surface). (Figure 3). The top of this facies at 2970 m is marked by an obvious angular unconformity. The tuffs below the unconformity are washed out down to a depth of approximately 2980 m and this is probably a result of weathering below the unconformity. This facies does not demonstrate any reservoir potential being comprised of 100% tuffs with no effective porosity, only in some interval which indicates fracture porosity.

Conglomerate Facies

This facies is classified as quartz granule conglomerate (sample depth 3051.0 m) and sandy conglomerate (sample depth 3062.5m). Conglomerate facies has granule to coarse sand in grain size. It has grain sizes mean 4 mm, poorly sorted, rounded – sub rounded, and observed domination of point contact type. The grains composition is dominated by rock fragments such as sedimentary rock fragment (quarzite and poly crystalline quartz) and also individual quartz grains with their sizes range from sand size up to > 4mm. Sand size quartz grains exist as in matrix mixed with argillaceous clay. The argillaceous clay matrix is probably consists of illite and kaolinite (Figure 4). The results of X-ray diffraction (XRD) analysis on 2 (two) samples of conglomerate facies in Jatibarang Formation demonstrate that the rocks consist predominantly of quartz (89 – 96%), with minor pyrite (trace), illite (trace – 1%), and kaolinite (4 – 10%). (Figure 5).

The diagenetic events occur in this conglomerate fasies are dominated by quartz overgrowth, followed by replacement of feldspar by kaolinite and illite (Figure 5), and fracturing process to form secondary porosity. Petrography observation under the microscope and scanning electron microscope (SEM) examination show the rocks in this facies have moderate porosity values (11.00 – 15.00% by volume). It is represented by intergranular and secondary fracture porosity types. Interconnectivity among pores is relatively good. Based on petrography analysis, the depositional environment of the conglomerate facies of Jatibarang Formation can be interpreted deposited in sub aerial alluvial plain (braided channel).

Based on FMI, this facies show that mixture of massive and cross-bedded sandstones, pebbly sandstones and conglomerates, and are best interpreted as stacked braided fluvial channel deposits. Cross bedding below 3058 m indicates palaeoflow to SW. The upper part of this braided fluvial section, above 3050m, is characterized by finer grained laminated and massive shaly sand indicating channel abandonment and the demise of fluvial deposition.


FORMATION CHARACTERIZATION OF VOLCANIC RESERVOIR

Formation characterization of volcanic reservoir at “Ratu-1” well is defined as the description process either qualitative or quantitative by using the available data (petrography, sidewall core, FMI, and petrophysic analysis). Based on available data, the volcanic rock is classified into volcanic clastic because it has been mixed with the sedimentary sediment. Integrated study of petrography, core, and FMI has been done above, so the author has to compare it with electrofacies and well log analysis to get the reservoir characterization both quantitatively and qualitatively.

Petrophysically derived porosity must be tuned to the volcanic clastic reservoir and matrix are generally required to be variable as the rock type changes. This volcanic clastic reservoir of “Ratu-1” well divided into three facies and they are confirmed by cross plot of RHOB and TNPH at Jatibarang Formation which are make cluster for each facies: massive tuffaceous sandstone, laminated tuffaceous sandstone, and conglomerate facies. Based on cross plot, “Ratu-01” well, there are three group facies which are show the different characters of well log value.

The formation of the Jatibarang volcanic rock created complex geological phenomena in both lateral and vertical directions. These conditions cause difficulties in making geological descriptions which represent the development of each volcanic lithology. The lithology complexity influences the physical properties of the rock. A study which is related to reservoir requires a basic reference parameter that is generally applicable to all types of volcanic lithology.

The author performs petrophysic calculation based on integrated study of petrography, core, and FMI parameters, in order to get the value of porosity and permeability from log and also is compared to petrography and core porosities. (Figure 8 ; Table 2). The parameters are expected to be a referencefor the petrophysics calculation in the other wells among the study area.

Reservoir characterization was carried out after look distribution of each facies in horizontal. The porosity influences on the quality of the reservoir based on classification of Koesoemadinata, (1978).Reservoir quality of Jatibarang formation, "Ratu-01" well in each facies based on classification ofcan be seen in Table 2. According to reservoir quality from Koeseomadinata (1978), the best reservoir is conglomerate facies which is sitting directly on the basement. It is a series of coarse grained clastics that are 17 m thick in the “Ratu-01” well. Koesoemadinata (1978)


CONCLUSION

From the study that has been conducted, several main conclusions can be derived.
1.   The lithology complexity of volcanic are influence the physical properties of the rock.
2.   Integrated study of petrography, SEM, and XRD are needed on Jatibarang Formation and compared with FMI and well log to identify reservoir characterization.
3.  Based on integrated study of petrography, core, XRD, FMI, and well log, and also is compared to well log analysis and electrofacies. The author is divides this volcanic reservoir of “Ratu-1” well into three facies; massive tuffaceous sandstone, laminated tuffaceous sandstone, and conglomerate facies.
4.   Based on petrographic analysis, reservoir quality of massive tuffaceous sandstone facies have porosity value = 2%, and conglomerate facies have porosity value = 11-15%.
5.   Based on petrophysic calculation, each facies has a different petrophyisic parameters such as;
a.   Massive tuffaceous sandstone facies (RHOB = 2219 – 2654 kg/m3; TNPH = 0.22 – 0.58 %; GR = 52 – 108 GAPI; DT = 239 – 342 usec/m; F = 9%; permeability = 3.08 mD).
b.  Laminated tuffaceous sandstone facies (RHOB = 2219 – 2654 kg/m3; TNPH = 0.22 – 0.58 %; GR = 52 – 108 GAPI; DT = 239 – 342 usec/m; F = 8.8%; permeability = 1.474 mD).
c.   Conglomerate facies (RHOB = 2219 – 2654 kg/m3; TNPH = 0.22 – 0.58 %; GR = 52 – 108 GAPI; DT = 239 – 342 usec/m; F = 13%; permeability = 82.381 ).
6. According to reservoir quality from Koeseomadinata (1978), the best reservoir is conglomerate facies. This facies has granule to coarse sand texture in grain size. The grains composition is dominated by rock fragments such as sedimentary rock fragment (quarzite and poly crystalline quartz) and also individual quartz grains and sand size quartz grains exist as in matrix mixed with argillaceous clay (illite and kaolinite).
7.  The Jatibarang volcanic reservoir in this study area is only compatible for this case, and can not be applied for other fied because the volcanic reservoir is very unique. It is need comprehensive analysis.


ACKNOWLEDGEMENTS

We would like to thank PT Pertamina EP Region Jawa and BPMIGAS for their permission to publish these data. We are acknowledgement IPA technical committee for accepting this paper to be published and the IPA reviewer for improving the manuscript.


REFERENCES

Arpandi, D., and Patmosukismo, S., 1975, The Cibulakan Formation as One of The Most Prospective Stratigraphic Units In The North-West Java Basinal Area: Proceedings Indonesian Petroleum Association, 4th Annual Convention, p. 182.

Kalan, T., Sitorus, H.P., and Eman, M., 1994, Jatibarang Field, Geologic Study of Volcanic Reservoir for Horizontal Well Proposal: Proceedings Indonesian Petroleum Association, 23th Annual Convention, p. 230-231.

Cholidy, H., Reminton, and Unggul, P., 1985, A Hydrocarbon Generation Analysis in Northwest Java Basin Using Lopatin's Method: Proceedings Indonesian Petroleum Association, 14th Annual Convention, p. 121.


2 komentar:

  1. Dear Zeindra Ernando,

    I would appreciate if you could contact me about this paper

    BalasHapus