Seismic sedimentology of a broad, low-relief carbonate platform: The Cambrian Longwangmiao Formation, Moxi–Gaoshiti area, Sichuan Basin, China

Abstract

Broad, low-relief carbonate platforms are commonly relatively flat lying and show minimal topographic relief. These characteristics make the platforms difficult to interpret using seismic data. To systematically analyze these platforms for reservoirs, a seismic-sedimentological workflow was implemented to investigate the paleogeomorphology and reservoir quality of the Cambrian Longwangmiao Formation (LWM) in the Moxi–Gaoshiti area of the Sichuan Basin, China.Conventional core and wire-line log data indicate that carbonate lithofacies are composed of completely dolomitized grainstones, packstones, and wackestones to mudstones. Associated depositional environments were interpreted as shoal complex (shoal crest, shoal margin, and intershoal), lagoon, and deep shelf. The low (4%–8%) porosity reservoirs are largely related to the shoal complex facies. Two third-order sequences were correlated throughout the field area, each demonstrating an upward-shallowing trend. The reservoir-quality rocks were formed in the highstand systems tracts of the sequences.Lacking diagnostic shallow-water indicative seismic facies (e.g., the topset of clinoforms), seismic paleogeomorphology was assessed by restoring the paleostructure at the top of the LWM and then evaluating the gross thickness of the LWM to establish the accumulation rate of sediments across the area. This approach demonstrated a fair correlation to measured reservoir-thickness data from wire-line logs, indicating that the paleogeomorphology and depositional facies were largely controlled by a syndepositional, en echelon fault system that determined the distribution of shallow-water shoal complexes and deeper shelf areas across the field area. Seismic lithology determination by amplitude analysis of multiple frequency panels provided a quantitative assessment of reservoir distribution, supplementing the qualitative paleogeomorphologic maps for field development and reservoir modeling.