The paper presents the developed methodology for building simplified reservoir models for integrated asset models (IAM) of oil and gas fields: allocation and substantiation of areas, substantiation of model parameters, substantiation of actual weighted average reservoir pressure for areas, history matching and validation, evaluation of effective injection factors, integration in an IAM, prediction calculations, model updating. The novelty of the methodology is the developed approaches and methods of considering different features of fields with a high extent of automation for areas and fields as a whole. Models based on the material balance method and two-dimensional proxy models of one-phase flow in porous media are used as simplified reservoir models in the paper. The developed methodology has been successfully tested for four oil and gas fields of Russia, which have different geological and production features: a large field with a long development history and a large number of active wells, a field with low permeability in all pay zones and high scopes of new wells commissioning, a field with a gas cap and high gas/oil ratios (GOR) for individual wells, a field with a complex system of reservoirs and tectonic faults and a large number of multi-pay production wells. For three out of four fields, at the moment, the IAMs have been transferred to commercial operation based on the pilot projects performed and are used by field specialists to solve the following problems: quality analysis of reservoir pressure measurements; assessments of actual reservoir pressure trends by areas; assessments of ineffective injection for areas; prediction of reservoir pressure, water cut and GOR profiles for wells (up to one year) for various prediction scenarios, including optimization scenarios (taking into account the limitations of the material balance method).