The Main Distribution Zone And Reservoir Forming Mechanism Of Lithologic Reservoirs In Oil-bearing Fault Sags, Erlian Basin

In Erlian basin, it is Mesozoic small fault lake basin group, where lithologic oil reservoirs are feasibly developed due to proximal and polyenic deposit, fast accumulation and quickly varied sedimentary facies belts. And in recent years, reserves of lithologic oil reservoirs have been over 70% of annually submitted reserves with the further study and exploration. Therefore, it is necessary to study lithologic accumulation conditions, sandbody control factors, reservoir forming mechanism and enrichment factors of the oil-bearing fault sags in Erlian basin, so as to and provide theoretical and practical basis for the exploration of lithologic oil reservoirs in Erlian basin and the similar fault sags.This paper makes sufficient use of logging , 3D seismic and sedimentary reservoirs and organic geochemical data. And according to the concept of sedimentation controlling sandbodies—traps controlling oil reservoirs—main factors controlling enrichment, this paper systematically compares hydrocarbon accumulation conditions in different zones (or blocks), analyzes forming conditions of sedimentary sandbodies, and studies reservoir forming mechanism of lithologic oil reservoirs and the dominant factors controlling oil reservoir enrichment. And the following understandings are consequently obtained.1. This paper conducts detail comparison on hydrocarbon accumulation conditions of 5 oil-bearing fault sags in different zones (blocks) in Erlian basin. And then it collates and stipulates the concept of trough area according to development features of the lithologic oil reservoirs in these fault sags. This paper also lineates the range of trough area, including the flank of steep slope belt, the trough zone and the middle and lower parts of gentle slope belt. The trough area is located at the development zone of weak structures, presenting developed source rocks and encircling or interlayering contact between reservoir sandbodies and source rocks. Obviously, it is an integrated reservoir forming unit.2. This paper studies the forming conditions of sedimentary sandbodies in the representative fault sags by means of sediment source, palaeoravine and slope breaking belt in different zones. And then it presents that distribution of sedimentary systems is controlled by palaegeomorphology, transportation pathway of sandbodies is controlled by palaeoravine and their superimposition relationship is controlled by slope breaking belt. It is indicated that the distribution of sandbodies are controlled by sediment source-pathway-slope breaking belt. The slope breaking belts in Erlian basin are classified into two categories, seven types. The superimposition relationship of sandbodies is controlled by the configuration of slope breaking belts.3. This paper systematically analyzes accumulation mechanism of 12 available lithologic oil reservoirs in terms of their trap formation and hydrocarbon migration, accumulation and preservation. The trough areas present favorable conditions for hydrocarbon accumulation, including early formation of lithologic traps, primary migration or short-distance secondary migration, early charging-early accumulation and favorable preservation. And based on these, this paper establishes four typical lithologic accumulation models.4. It is indicated from above-mentioned analysis that the enrichment of lithologic oil reservoirs in fault sags in Erlian basin is controlled by the following five factors, including main trough area with abundant petroleum resource, internal or proximal combination, favorable structure-lithofacies belt, predominant migration pathway and favorable reservoir sandbodies. Therefore, it is necessary to analyze the five factors of fault sags with multiple troughs (trough areas) and attention shall be paid on the latter three factors after enriched fault sags have been ascertained in the main oil-generating trough.