Integration of Geophysical Description Technique and Application to a Volcanic Reservoir - A Case from Permian Fengcheng Formation in Xia-72 Well Area, Northwest Margin, Junggar Basin*

Yongbo Chen¹, Jianguo Pan¹, and Duonian Xu¹

Search and Discovery Article #10307 (2011)

Posted March 14, 2011

*Adapted from extended abstract prepared for a poster presentation at AAPG Convention, Calgary, Alberta, Canada, September 12-15, 2010

¹Research Institute of Petroleum Exploration & Development-Northwest (NWGI) of PetroChina, Lanzhou, China ([email protected])

Abstract

Many volcanic reservoirs have been found all over the word since the end of the 19^th century. The lithologies of volcanic reservoirs are dominated by basalt, andesite and rhyolite, and the reservoir space has mainly fissures. However, the lithology of volcanic reservoirs in Xia-72 well area is lava tuff breccia and the reservoir space is stomas. Considering the particularities of the lithology and reservoir space, the characteristics of the superior reservoirs were analyzed, geologic forming models were developed and locations of superior reservoirs were predicted using geophysical techniques guided by the geologic models.

Figures

	Figure 1. Well Xia-72 core photo.
	Figure 2. The model of fissured effusion.
	Figure 3. The forward migration section of low speed.
	Figure 4. Coherence slice (3450 ms).
	Figure 5. Paleogeomorphology of the volcanic eruption.
	Figure 6. 3D distribution of volcanic reservoir.
Characteristics of the Volcanic Reservoirs The lithologies in the bottom of the Permian Fengcheng Formation are dark grey tuff, rhyotaxitic lava tuff breccia and breccia. Rhyotaxitic lava tuff breccia, which was found in Well Xia-72 (Figure 1), is friable, hard and foamed in hot acid. The rhyotaxitic lava tuff breccia has fluidal structure, brecciated texture, and partly felsitic texture. Stomas are predominant and irregular, and in different sizes. Crystalline quartz developed in the stomas, and the connectivity between stomas is poor, although the connectivity could be better after tectonic movements and fracturing. Geologic Forming Models of Volcanic Reservoir The Permian Jiamuhe Formation and Fengcheng Formation were deposited in the foreland basin setting. Volcanic eruptions occured in the northwest margin under the weak compaction and short-time laxztion. A distinct volcanic structure exists in the lower and middle Jiamuhe Formation in the Xia-72 well area: a moundy volcanic rock complex, including volcanic channels and explosive and effusive volcanic rocks. During late Jiamuhe Formation, volcanic activity was in a dormant period with one pyroclastic rock construction: grey dolomitic and tuffaceous mudstone, dust color and brown sand-shale and black tuff. At the end of Jiamuhe Formation and early Fengcheng Formation, one rock construction, which includes tuffaceous fine sandstone, siltstone and mudstone rhyotaxitic lava tuff breccia, developed. In the early Fengcheng Formation, layered volcanic rocks developed during the early period of volcanic construction, which effused along the Xiahongnan Fault (Figure 2). The volcanic eruption has two periods, the early and middle Jiamuhe Formation was centered eruption, and the early Fengcheng Formation fissured eruption. The lava tuff breccia was from fissured eruption and belongs to volcanic clastic stream subfacies. Volcanic Rock Reservoir Integrated Geophysics Descriptive Techniques Forward Technique The logging response of the lava tuff breccia is characterized by low-speed, low-density with a more continuous, strong trough seismic response in Fengcheng Formation of the Xia-72 well. According to this character, and combined with "wave-related" technology, a strong continuous trough is found at the Jiamuhe Formation top. The forward technique is used to verify whether the anomalous body is the seismic response for the lava tuff breccia. Firstly, the anomalous body high speed geologic model is set up by combining the acoustic velocity and density data with a seismic structural model. The seismic section obtained by using forward technique on that model does not conform to the actual one on amplitude and phase characteristics. Then the low speed geologic model is established, which also used the forward technique to gain the seismic section. Coincident with the actual profile (Figure 3), which shows that the anomalous body is welded tuffs response and also endow with geological implication useful to high quality reservoir prediction. High Resolution Coherence Technique High resolution eigenvalue coherence algorithm is adopted to identify the plane distribution for eruptive faults and craters. The coherence algorithm needs to consider both the waveform analogy and wave directivity owing to using eigenvector on this method. The interpretation for the horizontal section from this technique (Figure 4) not only depicts the main fault and crater plane distribution, but also shows the moniliform allocation on the plane along the Xiahongnan Fault. Every crater is characterized by “Cabbage” on plane. Palaeogeomorphology Recovery Technique Pyroclastic flow facies are closely related with the volcanic eruption palaeogeomorphology. Volcanic materials from eruptions of the fissure type flow to the lowland along the fault. The common palaeogeomorphology recovery technique used now includes residual thickness and compensating cast method, repay and filling renew, sedimentary analysis and sequence stratigraphy recovery (including high resolution sequence stratigraphy). The volcanic eruption interacted with clastic deposition during the Jiamuhe and Fengcheng formations during the Permian. Therefore, the top boundary of clastic deposition is the sequence surface, which combined with the adjacent volcanic rocks thickness to acquire the palaeogeomorphology (Figure 5). As shown in the figure, topography fluctuated greatly, being high in the south and low in the north. The paleogeomorphic lowland along the crater contains the pyroclastic flow facies. Prediction Technique of Filling Degree of Stomas Vocanic rocks were found in the Fengcheng Formation of Xia-76, Xia-88 and Qi-8 wells. In the Xia-76 and Xia-88 wells the lava tuff breccia with the semi-filled stomas got stripper oil flows. Qi-8 had a poor reservoir porosity because its stomas were fully filled. Through geologic analysis, we concluded that the storage space of the lava tuff breccia was mainly “primary pore”. The later stage tectonic movement caused the development of faults and surface water which contained SiO₂, which seeped into the tuff through faults and fissures and filled the primary stomas. The filling degree of stomas using forward modelling was studied and we found that seismic response characters were high energy and high amplitude when the stomas were not filled, medium energy and amplitude when the stomas were semi-filled and low energy and amplitude when the pores were fully filled. From the correlation of filling-degree and amplitude values, we could see that pores had the maximum amplitude with a value of 120 and horizon velocity of 4300 m/s when they were uncharged. 3D Visualization Technique Based on the different reflected amplitude of rocks, we adjusted the background to opaque and made the main geologic objects transparent to highlight them. When adjusting the parameter we studied the seismic response characters first. For example, the high grade reservoir (developing ignimbrite breccia tuff and stomas) at the bottom of the Fengcheng Formation in Xia-72 well was demarcated as a high amplitude trough in seismic section, and so this high amplitude trough stifled the medium and low amplitudes and the transparent curviform highlighted the high amplitude. Figure 6 shows the distribution of high grade reservoir at the bottom of the Fengcheng Formation in Xia-72. The high grade reservoir distributed from south to north as a flow with a distance of 7.5-8.2 kilometres, which coincides with the palaeogeomorphology. Application Analysis In the Xia-72 well area, using the predictive thinking and methods for fissure-type eruption of volcanic high grade reservoir, we predicted an area of 243.1 Km², and petroleum resources of 7685×104 t. At the same time, according to matching between tectonics and high grade reservoirs, we located the Fengnan 4 well and discovered commercial oil and gas flow at the bottom of the Fengcheng Formation from lava tuff breccia and developed stomas. Conclusions Overall, by analyzing the origin of the high quality reservoirs of volcanics, under the direction of the geologic models, we conducted precision prediction on those high quality reservoirs of volcanics by performing various geological and geophysical techniques: (1) By analyzing the features of the reservoirs of Xia-72 well volcanics, the high quality reservoirs (vesicular lava tuff breccia ) belongs to pyroclastic flow facies, which is the product of fissure eruption, and built a fissure eruption model. (2) The distribution of pyroclastic flow facies is mainly controlled by the early crater and the palaeogeomorphology before volcanic eruption. By using the high resolution coherent technique and palaeogeomorphology recovery technique, we carried out the prediction for the distribution of the pyroclastic flow facies and considered that pyroclastic flow facies mainly lies in those zones close to the crater and low-lying palaeogeomorphology. 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