Paleogene Volcanic Rocks In Eastern Depression Of Liaohe Basin: From Petrogenesis To Exploration

Liaohe Basin located in the transitional zone of Tan-Lu fault. It is one of the most important Cenozoic and Mesozoic petroliferous basins in China and the hot place where geologists studied in the past few decades. The eastern depression of Liaohe baisn is one of the dustpan-like depressions with NE trending. Paleogene strata in Liaohe basin are composed of Fangshenpao formation, Shahejie formation and Dongying formation. There are large quantities of volcanic rocks within the strata, this set of volcanic rocks not only thoroughly documented the Paleogene volcanism progress but also the major oil-gas reservoir in this area, so by comprehensive and systematic study on them will play a very important role in understanding evolution and dynamics process of Paleogene structures in Liaohe and Bohai Bay basins, as well as practical exploration and application of oil and gas. Therefore, based on volcanic distribution characteristics, in this paper we have studied lithofacies of volcanic rocks, petrogeochemistry, isotope geochemistry, petrogenesis, major controlling factors of reservoir, evaluation criteria and physical property cut-off value; analyzed Zircon U-Pb dating, major and trace elements, Sr-Nd isotope, porosity and permeability; discussed volcanic rocks characteristics in source area, forming process and tectonic background. On that basis, according to the features of structure and reservoir physical properties, we have carried out studies on reservoir physical properties of volcanic rocks, discussed development characteristics, controlling factors and physical property cut-off value of volcanic rocks as a major oil and gas-bearing reservoir, and developed classifying and evaluating criteria especially for volcanic rocks in Liaohe basin. The main conclusions and understandings are as follows: 1. Combination of volcanic rocks and discovery of dioritic porphyrite.Paleogene volcanic rocks in the studied area are mainly composed of intermediate-basic basaltic rocks and trachyte as well as their symbiotic subvolcanic rocks, generating no acidic rocks. Volcanic rocks can be divided into 4 types according to the rock structure, namely volcanic lava, volcanic clastic lava, volcanic breccia and subvolcanic rocks, further divided into 12 subtypes respectively are basalt, basaltic andesite, andesite, trachyandensite, trachyte, brecciated trachyte, tuff lava, breccia lava, tuff, volcanic breccia, diabase and dioritic porphyrite. Dioritic porphyrite is commonly regarded as trachyandensite emerging from a volcano in previous studies. But by comparing trachyandensite in the center and north of the studied region, the author find that dioritic porphyrite have the characteristic of stratified structure or crossing structure with a higher degree of crystallinity, smaller extension in longitudinal and lateral directions, and poorer continuity of distribution. And according to measurement, its zircon U-Pb age is 31.47±0.35 Ma, visibly younger than the 3rd member of Shahejie formation(42.4~38Ma), but at the same stratigraphic age as overlying Dongying formation(32.6~24.5Ma). Based on the above characteristics, it is not trachyandensite emerging in the epoch of the 3rd member of Shehezi formation but dioritic porphyrite generated by intrusion and belongs to subvolcanic rock. 2. Geochemical characteristics and petrogenesis of volcanic rocks.There were basic volcanic rocks developed in each epoch of Paleogene in this area, but they have different geochemical characteristics. Major elements contained in basic volcanic rocks in Fangshenpao foramtion are characterized by low-alkali and low potassium, where K2 O account for 0.08%~0.15%, and Na2O+K2O account for 3.72%~3.79%. The total content of rare earth elements is low, and there is no significant fractionation in the mixture of rare earth elements, with loss of large ion lithophile elements and a lower εNd(t) value =1.58. And the source region of this formation is an asthenospheric mantle affected by subducted oceanic slab flow. The basic volcanic rocks in the following 3rd member of Shahejie formation, 1st member of Shahejie formation and Dongying formation are rich in aluminum and sodium, where Al2O3 accounts for 16.08%~18.36%, and Na2 O accounts for 2.33%~5.04%. Strong differentiation is found in VII the mixture of rare earth elements with weak enrichment of large ion lithophile elements and enrichment of high field-strength elements such as Nb and Ta, where ISr=0.7033~0.7042 and εNd(t)=3.56~5.86, sharing geochemical characteristics and isotope values similar to those of OIB. Their source region is also an asthenospheric mantle but it is under weaker effects of oceanic slab flow, instead, more enriched components in upper mantles join. Basaltic magma of Fangshenpao group and 3rd member of Shahejie formation section was not composed of primary mantle magma, but formed by fractional crystallization of monoclinic pyroxene and apatite in the primary magma. Basaltic volcanic of 1st member of Shahejie formation section and Dongying formation can represent components of the primary magma and they did not experience fractional crystallization. Basaltic volcanic was not obviously contaminated by continent crust in forming in all epochs.Trachyte volcanic rocks and subvolcanic rocks of dioritic porphyrite are recognized in 3rd member of Shahejie formation. Trachyte Si O2 content percentage among 59.84%~64.04%, rich in aluminum and potassium, Na2 O content percentage among 2.98%~6.19%, K2 O content percentage among 3.95%~9.28%, ratio of K2O/Na2 O larger than 1, belong to potassic volcanic rocks. The curve variation tendency of rare earth elements and trace elements is in line with that of basaltic volcanic over the same epoch, and have similar isotope values ISr=0.7035~0.7045, εNd(t) =3.25~4.46, which indicating that the both have the same source. Significant negative anomaly of Eu and losses of Sr, P, and Ti imply that trachyte experienced fractional crystallization of plagioclase, apatite, rutile, and ilmenite, etc. The curve shape of rare and trace elements between dioritic porphyrite and basalt is similar. The dioritic porphyrites nearly have no negative and positive abnormality of Eu, obviously loss of P and Ti, indicating that magma might experience fractional crystallization of apatite and ilmenite and no contaminated by crustal matters during its forming progress. 3. Tectonic setting of volcanic rocks.Active continental margin areas are widely thought to be the most complex tectonic environments on the earth. Paleogene is the time of Pacific plate west side having changes in subduction direction and rate, which made the tectonic environments in Liaohe basin became more complex. Geochemistry and isotopes have shown such similar characteristics of different environments and the rocks developed in different formation of VIII Paleogene have also shown completely different characteristics. There is a small gross amount of rare earth elements in volcanic rocks in Paleogene Fangshenpao formation and there is weak fractionation in the mixture of rare earth elements with low content of trace elements and loss of large ion lithophile elements Ba and K, showing similar characteristics to those of MORB. From 3rd member of Shahejie formation section including the following 1st member of Shahejie formation section and Dongying formation, the geochemical characteristics are completely different from those of Fangshenpao formation. First, the gross amount of rare earth elements obviously increases, differentiation in the mixture is greater and the content of trace elements is also higher than volcanic rocks of Fangshenpao formation. Second, the geochemical characteristics and isotopes are similar to those of OIB, but compared with typical OIB, they have a lower content of incompatible elements in general with weak loss of Nb and relative enrichment of K.According to further analysis of geochemical and isotopic characteristics of the Paleogene volcanic rocks in this are, Liaohe basin was considered under an intraplate tectonic background in Eocene and Oligocene epochs of Paleogene and in Paleocene Liaohe basin began moving from the active continental margin to an intracontinental rift. Volcanic rocks were formed in a back-arc extension like tectonic environment, and changes in Paleogene tectonic environment in this area was mainly controlled by changed subduction direction and rate of the Pacific plate. In the Mesozoic era, the Pacific plate began to subduct to the Eurasian continent. In the Paleogene, subduction rate slowed down and the oceanic slabhad subducted into asthenosphere sank down due to gravity, which pushed the subducted slab to draw back and also drove the fosse to retreat to the ocean, so that the continental margin was under tensile stress, resulting in tension of the back-arc area. In this context, the lithosphere was subjected to tension and asthenosphere started to rise, so that lithospheric thinning would appear in the depth with and due to delamination. At the same time, Tan-Lu fault system got activated with large-sized strike slip motion, resulting in volcano-magmatic activity of the mantle source of Liaohe basin over the same epoch. 4. Significance of volcanic rock reservoir.Effective volcanic rock reservoirs are mainly controlled by faults, eruption cycles, and lithology. Faults control spatial distribution of volcanic rocks and development of secondary fractures. Volcanic rock reservoirs are intensively developed along large-sized strike slip faults, especially at intersections of main faults and derived fractures, and penetration rates of rocks are mainly determined by development level of secondary fractures. Eruption cycles control longitudinal distribution of volcanic rock reservoirs. In this area there are 3 volcano eruption cycles developed from bottom to top, including 14 stages, and the effective reservoir are intensively developed at trachyte stages(S3q3) in the middle of Cycle II. According to lithological combination and position forms, volcanic facies in this area are divided into 5 types respectively are volcanic vent facies, erupting facies, flowing facies, extrusive facies and volcanogenic sedimentary facies, and it can be further divided into 14 subfacies. Volcanic facies control the size of reservoirs and development zones of primary pores and fractures. In this area, 4 subfacies including pyroclastic flow subfacies of erupting facies, outer, middle and inner zones of extrusive facies are places where effective reservoirs were intensively developed. Lithology determines reservoir space types and degree of later alternation and transformation. Reservoir space includes primary and secondary types, which can be divided into 9 types and 14 subtypes according to formation mechanism, morphology and distribution characteristics of the reservoir space. Primary reservoir space refers to various open pores and fractures, composed 4 types including primary pores, grillage pores, contraction fractures and spalled fracture, formed in airtight conditions before volcanic rocks completely cooled down under the primary diagenesis. Secondary reservoir space was formed in open conditions after volcanic rocks completely cooled down, coming from transformation of primary reservoir space or formation of new space under the secondary diagenesis, composed of 5 types including dissolution pores, concealed erupting fractures, weathering fractures, dissolution fractures and tectonic fractures.Effective reservoir refers to those have industrial value for liquid yield produced with the existing technology and under the current economic conditions. By using the porosity, permeability, oil-bearing occurrence, formation test results and other information to determine the physical property cut-off value of volcanic rock reservoirs in this area and develope criteria for reservoir classification and evaluation. Physical property cut-off value is calculated respectively by empirical coefficients method, minimum oil-bearing throat radius method, oil-bearing occurrence law method and formation test method. According to these four methods, physical property cut-off value of volcanic rock reservoirs in this area are determined as 3% of porosity and 0.03×10-3μm2 of permeability. If porosity lower than 3% or permeability lower than 0.03×10-3μm2, industrial or low-yielding oil and gas reservoirs cannot be formed in general. According to physical properties of reservoirs, volcanic rock reservoir can be divided into 4 types: Type I is high porosity and high permeability reservoir(Φ>13%, high permeability K>0.4×10-3μm2); Type II is relatively high porosity and relatively high permeability reservoir(Φ=13~8.5%, K=0.4~0.1×10-3μm2); Type III is medium porosity and permeability reservoir(Φ=8.5~3%, K=0.1~0.03×10-3μm2); Type IV is low porosity and low permeability reservoir(Φ<3%, K<0.03×10-3μm2). Oil reservoirs are mainly developed in type I and type II.