The Science Report to participate in the Science Conference

I. GENERAL FEATURES

During the recent decades, the reef formation has been known as playing an important role in petroleum reconnaissance because of its very good ability of storing hydrocarbon. This article aims to introduce in details the geological, geophysical and physico-chemical methods that have helped us in discovering the oil-bearing reef at the drilling well Dx in Hx field. When testing, this zone has given good production (the reservoir is only 5.2 m thick, but it gives the productivity of 4,200 barrels/day). In this article, the authors present the results of study on the reef formation and its significance in the petroleum prospecting and exploration works.

II. GEOLOGICAL MOSAICS OF REEF

The term “reef” is commonly used for production of carbonate secreting organisms built in the marine environment. There are some kinds of reef, such as “parch reef” and “round reef”, also called as atoll.

1. Creation of round reef.

The round reef type is usually seen in the shallow-sea area with waves and temperature at about 20°C; it is composed of “coral reef”, “alga reef”, “moss reef”, etc. (Fig. 1, 2). They are diversified in organism species, creation form, space and time. About space, they exist almost everywhere in limestone. As the separate structures, they are scattered in the shallow limestone area, or further, to the slope side and to the ocean crater. About time, they have been existing from the Cambrian age until now. Because they are the individuals attached together, they often create long immerged patches on the continental shelf and continental slope, surrounding the land.

The essential thing for the lime organism to create reef is to get the large Metazoa frame (multicellular creature). This frame always grows; it is developed rapidly, but perishes early. The next generation grows right on the body of their parent one (the modern coral reef is an example). N.P James (1983) believed that if we want to create reefs, Metazoa must have firm foundation for them to cling on to and grow. After perishing, Metazoa often remains on the premises unless it is too weak; it may be defeated and swept away by waves. Finally, the very large, empty, cave frame is created. In such large caves, there are the small lime organism species, which secrete lime mud, or parasitize-corrode reefs and secrete lime mud, grow and perish right in Metazoa. All created the “inner” sediment material of reefs. So that the caved level of Metazoa may be not as large as before. Anyhow, reef is still the kind of limestone with larger emptiness and endosmosis in comparison with any other sedimentary stones. The general feature of the organism species, which create reef, is that they live off the calm food filtering apparatus. Therefore, their food filtering apparatus will be stuck if there are fine-grains. On the other hand, there are always caves inside Metazoa, so the fine sediment material accumulates easily in reef. That is the reason why we often meet in reef the other fine sediment materials beside carbonate.

The other important thing is that reefs do not exist or become perished in the muddy water areas as the estuaries and the areas with the saltiness that changes very often, such as flooding areas, areas penetrated by fresh-water. The Metazoa’s diversity also affects greatly the development of reef. That diversity depends totally on the environment condition. Organisms develop well in the optimal growing condition, which means the

sufficient food source and less change in physicochemical condition within a day. The elements that affect the organism life are:

- Change in saltiness and temperature: They develop well in tropical sea (from about latitude 30° South to 30° North), warm water (about 20°C) and the stable saltiness within 2.7-4 %.

- Intensity of wave and stream flow: Only the strong frames can exist.

- Lighting level: The calcifying process occurs rapidly in modern reef, because its surface is covered by the symbiotic organisms that live off the light. On the other hand, the deeper the weaker of light intensity, only less species can exist. Coral cannot exist at the depth of 75 m.

- Accumulation: All the organism species are calm food filters or the super small carnivorous.

2. Facies of Reef

There are a lot of reef-creating forms and many reef facies released by the authors so far. However, they are only different in name-calling way. So it’s possible to gather them in three major facies as follows:

a. Reef-core facies: Stone is not divided in layer. It is in massive structure with accumulated Metazoa on spot together with sedimentary materials. They are mainly massive, forming limestone lenses containing skeletal frames and lime mud foundation. Reef-core facies is developed in high-energy zone. Wave energy takes fine sediment materials out where mud is almost absent. Large and stable Metazoa frames will create valuable reefs. There is still lime mud in reef because of “inner” sedimentary material as mentioned above. Reef limestone in this zone has good ability of storage; we call them as “store stone” or “store zone”. Reef-core facies has clear zoning.

b. Top zone: This is the highest zone of reef in all developing periods. Reefs at top zone inhale much energy from waves, wind, if they are in the shallow-water area. The composition of top zone reef depends on the intensity of waves and wind. Limestone has got forms of from bindstone to framestone.

c. Reef-flank facies: This zone spreads to the depth of about 100 m. The skeletons are developed strongly and turn slowly into the sediment material of the offshore reef area. The organisms are diversified in shapes, from semicircle to branches. They are mainly the shell faunas, such as brachiopods, bivalves, together with corals and crinoids. The stone in this zone still consists of framestone, because of the high level of the erodibility. Moving on further to the increasing depth, the light intensity decreases, organisms must increase their surface area resulting to their thin facies. Stone in this zone is as same as bindstone, but the “binding” phenomenon seems to be not important, because they contain a lot of debris eroded to from the top zone. The deepest area for the species as corals, green lime algae to develop on modern reefs may reach 70 m. The sediment materials of reef-flank facies include 2 kinds:

- The inner sediment materials, existing in reef structure. Normally, lime mud built foundation of mudstone and wackestone.

- Rough sand and gravel grit, pebbles have penetrated into reef on the way transporting away.

d. Inner-reef facies: Containing debri frames generated by the broken up top zone, together with sand, gravel stopped by the top zone-reef and strong developed algae in this sea area. This area contains lime sand that is quite clean. Organisms spread largely. Stones include kinds from grainstone to rudstone.

e. Reef-core facies: It is correlative with the high-energy area according to the model of Friedman and Sanders (1978). According to classification, the reef-core facies is autochthonous, without layer classification and fixed facies, but its emptiness and endosmosis is very high.

f. Reef-flank facies: The reef-flank facies always have fixed slope, including lime joined pebbles, lime joined sand coming from reef and getting thinner and thinner from the core. The reef-flank facies are correlative with the reef-secreting organism at slope flank (see Fig. 4). This is the transition area from the continental area to deeper sea area. Stone provides poor level of choice, due to multi-origin sediment materials and low diversified organisms: only some species can exist here. For us, the reef-flank facies exactly are “offshore reef facies”. They are developed at the continental slope in the high-energy zone, extending to the low energy zone and called in the past by some authors (Friedman and Sanders, 1978) as “ocean crater facies” or “deep sea facies”. These facies consist of rough firm layers of from grainstone to packstone, from thin to thick, all or only the debris frames, reef limestone blocks and the creating-reef skeletals. The feature of reef-flank facies is the more moving on further, higher depth; the stone tends to turn in to mudstone that means the more lime mud foundation is strongly developed, resulting a decrease in the emptiness, endosmosis of stone.

g. Burying facies: According to N.P. James, the burying facies in stone are not related to reef secreting organism, to be less affected by tide and developed in the shallow-water area. For us, the burying facies correctly are ocean crater facies or deep-sea facies, as stated above. This is the low energy zone in the model of Friedman and Sanders (1978). Offshore, sea-bed is not affected by waves, so it is very calm. The sediment materials are very smooth, even the debris of skeletal frames, and rich of organic secondary substance, that is transported from the “high energy zone”, of which, there are the ephemeral organisms of the “low energy zone” itself. Therefore, stone that is usually called as “mother” stone in this area generates hydrocarbon very well. Mudstone, marl, shale appear popularly in this area. Especially, there is the bacterium species existing in this area, which is capable of sterilizing sulfate. They synthesize the organic secondary substances into H₂S. The sediments in this environment have clear classification. The more moving on further, the more lime mud decreases. And the result is that shale no longer contains of lime mud and it is in dark colour. Besides, the low energy zone is also near to the seashore that is very shallow. The starting point from the water edge always changes. It depends on many factors, such as weather, tide, even on the periods of sea transgression or regression. Therefore, this area sometimes is

Figure 1. Creation of round reef.

large, but sometimes is narrow. The feature of this area is the presence of lime mud, tabular and skeletal materials. There are many species of algae and seaweeds in this area. The basic difference between this area and other ones is that, it is strongly oxidized due to the regular contact with the air. As a result, sediments lose their bedding and lime mud curdles in tablet (Friedman, 1981). The presence of dolomite and stone formed by evaporation also exists here. Stones in this area are often porous, that is capable to be the beds of the storage sediments of high-energy zone.

Figure 5. Reef development.

Figure 6. Graph presenting the results.

III. STUDY METHODS

The method used in this study include the combination of mud logging methods: physicochemical tests, Materlog of mud logging analysis, D_EXP, analysis of drilling technology parameter graphs; geological method: observation, description of drill cuttings, drill cores; observation of petroleum manifestations directly via the core sample collection; method of analyzing the wireline logging.

Figure 7. Graph of calcimetric analysis of reef.

1. Combination of mud logging study methods:

Reef zones manifest oneself through mud logging materials. For mud logging, many study methods are used, especially for the exploration wells. In this article, we mention only the methods that are directly related to the study and discovery of reef zone.

a. Visual analysis method of drilling mud samples: With the naked eyes, according to the analysis of drilling mud sample from the well DX under the microscope of ×10 enlargement, we have found the reef zone in the depth of 2295-2312 m. The drilling mud samples turn from marl 2283-2295 m, mixed with little of siltstone and fine-grained sandstone, poorly consolidated to limestone of mudstone-wackestone and some areas of packstone with average hardness (2295-2312 m), containing debris of corals, brachiopods, ostracods, crinoids; secondary minerals include a little of pyrite, mica, and little of siltstone and mostly fine-grained sandstone, some areas coarse-grained sandstone are met, the smoothness is from poor to medium, cemented by calcite. Under this bed, there is thick fine-grained shale cemented by calcite. This may be the firm foundation for reef existence and development (James, 1983). We consider that, this is the reef-core facies changing gradually to reef-flank facies. According to zone classification, they belong to the high-energy zone. Examining the full section in the field, we can see very clearly the reef development in width and thickness as well (Fig. 5).

Although there is no core within this interval, we confirm that the limestone in the depth between 2295-2312 m has got very good emptiness, because of oil smell and mostly direct and indirect cutting fluorescence very well (Fig. 6), which prove that they have got good storage capability (emptiness). Under the microscope, we cannot find any trace of oil, that means they have got good endosmosis.

The drilling speed within this interval highly increases, which is in conformity to the limestone layer or sandstone (Fig. 6).

The total and partial gas, especially gas from C3-C5, appears to be high in the depth of between 2295-2312 m, that is also in conformity to the emptiness considerations.

b. Physicochemical test methods on the spot:

A. Test with acid: Using HCl 10 %: there is differentiation between reef limestone and other stones; limestone effects with HCl 10 % strongly and quickly, and leaves transparent solution. Dolostone only effects partially with heated HCl 10 % (as same as siderite) and leaves milky solution. The surface of dolostone changes also in colour.

All cuttings in the interval of 2295-2310m, we think the reef limestone effects strongly and quickly with HCl 10 % and dissolved completely.

Figure 9. Drill cuttings.

Usually calcite reacts immediately and dolomite more slowly; the speed of reaction depends on the shape of stone, emptiness, endosmosis and impurity. The oil penetrating limestone has slow reaction due to the coverage of oil layer, only equal to dolomite. Clean carbonate stone is dissolved completely and sediments often are chert, anhydrite, clay, siltstone, sand and other secondary minerals. When effecting with acid, oil in stone will creates big bubbles surrounding stones.

- Acid engraving: Dipping the stone into acid; the carbonate stone will have a surface as polished, other stones, even dolomite, remain the initial state.

B. Dyeing method: Using Alizarin Red to dye the drilling mud samples in order to differentiate limestone and other ones, especially dolomite depending on calcite (CaCO3) is dyed in red colour, but other minerals do not change their colour. We have dyed most of cuttings that were in double to be dolostone and several fossils within the above said depth interval. As the result, they are all dyed in red. Only 1-2 too small cuttings do not change their colour or give unclear result, it may be dolomite mineral.

C. Calcimetry method: The basis of this method is because of carbonate content in sample basing on the usage of calcimeter to measure CO₂ pressure coming from the reaction of sample with HCl acid of 50 %. From this method, we can obtain the graph that can be identified under geoservice standard form: clean limestone, lime-bearing stone, oil- bearing limestone, in addition to graphs that allow us to identify pure dolostone, dolomite, lime dolostone, dolomitic limestone…etc. The Fig. 7 presents the graph of calcimetric analysis of reef.

Definition of CaCO₃ content in drilling mud sample by calcimetric method using HCl 50 % to differentiate limestone and other stones:

0-35 % CaCO₃: Clay

35-65 % CaCO₃: Lime clay - clayey lime

> 65 % CaCO₃: Limestone

If the sample is obtained accidentally during testing, CaCO₃ content only needs to reach from 50 % upward and we can assure that is limestone. Testing has been carried out accidentally at depth of 2295-2310 m. As result, all samples gave value CaCO₃>60 % (Fig. 7).

2. Method of Materlog of mud logging analysis and D_EXP

In the Materlog of mud logging methods at 2296-2312 m (Fig. 6), we see the drilling speed is 10 minute/m, rocketed up to 2 minute/m; the displayed values of ROP show us that the drill bit is coming in the spongy empty zone and appears the gas content (Tlgas: 42.8 %; C1: 18 %; C2: 2.5 %; C3: 2.1 %; iC4: 0.7 %; nC4: 0.5 %; iC5: 0.2 %; nC4: 0.17 %). The analysis of the gas rate: C1/C2, C2/C3 × 10 and the gas triangular diagram (Fig. 8) shows us that, this is the product reservoir. A quite important signal is that there is abnormal gas of CO₂ on reef formation. When drilling the carbonate formation, always appears CO₂content, but the abnormal gas of CO₂ on reef formation is still higher than medium CO₂ of the CO₂ in carbonate formation (Fig. 6).

On D_EXP graph: Reef zone shows unclearly. May be we don’t have much data so we haven’t found the rule yet, but we found primarily: the formation pressure and the formation destroyed pressure are very consistent (as a constant). The D_EXP value of reef is smaller than value of normal sediment formations or thick carbonate. We will collect more data of other drilling wells to clarify the manifestation of reef zone via D_EXP.

3. Geological analysis method of drill cuttings and core:

Through the drill cutting observation (Fig. 9) we see clearly that this is the typical reef zone-containing product. The drill cutting components consist of shell of gastropods, bivalves, foraminiferas, corals, cemented by lime mud, with direct and cut fluorescences, that are both clear with fast intensity.

IV. REEF ZONE SHOWN ON WIRELINE LOGGING

We see that the LLD and LLS values are the same and they are greatly different from MSFE value. This seems to express a caving zone of the drilling well, but the calibre of the drilling well diameter is highly consistent and close to the bit diameter (12”), proving that the drilling well diameter has no caving zone. Therefore, the penetrated area is a very strong penetrated zone with very high effect usage of porosity and permeability. This also shows through the value of lines RHOB, DT, PEF. One remarkable thing is the value of GR (gamma ray) of reef zone is as same as tuff value (volcano ash sediments). This is perhaps a new feature, signal added in the petroleum exploration for objects of reef- storage reservoirs. We can add one more feature of GR line for this matter that the very strong snatching amplitude at reef formation (though snatch at small value - Fig. 5).

V. CONCLUSIONS AND PROPOSALS

1. Oil-bearing reef formation is one of rare storage forms; the detailed study based on the geological and geophysical data is necessary for the petroleum exploration and exploitation in these special geological objects.

2. We would like to propose that from values of RHOB and DT in wireline log, we can calculate the apparent values: ∆RHOB = RHOB_Cr - RHOB_Form (Cr: pure carbonate, Form: Formation) and coefficient: D_VWLL= RHOB_Form / RHOB_Cr and value ∆DT = DT_Cr – DT_Form; DVDT = DT_Cr/ DT_Form.

3. In mud logging method: ∆Rop = ROP_cx – ROP_Form and D_VRM = ROP_Form/ ROP_cx with ROP_cx: drilling speed in thick carbonate; ROP_Form: in formation. Study to build the function: 1) F1 (Φ,Ψ) = K.RHO Reef (∆RHOB; DVWLL); 2) F2 (Φ,Ψ ) = K.DT Reef (∆DT; DVDT) in which K: experimental coefficient, Φ: effective usage of penetration; 3) To build the function F3 (Φ,Ψ ) = K.ROP (∆Rop; D_VRM). To make experiment to calculate root of these functions. To find the relationship of the compound function F (F1; F2; F3) = Φ (Φ,Ψ).

4. We name these functions are f (QT): proposed and taken the initiative by our group.

REFERENCES

1. General geological and geophysical documents, Materlog documents 69, 924, 450, 818, 702, 711, 301, 304, 303, 75, 705, 706, 76, 306, 308, 304, 1116, 14Dx and several other drilling wells on White Tiger and Dragon fields. Vietsovpetro, Vung Tau.

2. Lasen G., Chilingas G.V., 1967. Diagenesis in sediments. Elsevier Publ. Comp., New York.

3. Nguyen Cong Khac, Nguyen Duy Que, Do Quang Tien, 2004. Treating process of cutting, core. Arch. of Logging Enterprise, Vung Tau.

4. Nguyen Tuan Anh, 2002. Identify and discover the weathered zone, fracture of basement. Evaluate the storage capability of basement based on mud logging document. Petrol. Rev., 2.

5. Nguyen Tuan Anh, 2003. Application of off-line treating program in drilling technology. Petrol. Rev., 1.

6. The mud logging Process, 1995. File training # 11 Geoservices. Company. Paris. Translated and edited by Nguyen Hong Nam and Mud Logging Team.